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Simple New Keynesian Model 
       without Capital
      Lawrence J. Christiano
Outline
• Formulate the nonlinear equilibrium conditions of the 
  model.

   – Need actual nonlinear conditions to study Ramsey‐optimal 
     policy, even if we want to use linearization methods to 
     study Ramsey. 
      • Ramsey will be used to define ‘output gap’ in positive model of the 
        economy, in which monetary policy is governed by the Taylor rule.
      • Later, when discussing ‘timeless perspective’, will discuss use of 
        Ramsey‐optimal policy in actual, real‐time implementation of 
        monetary policy.  
   – Need nonlinear equations if we were to study higher order 
     perturbation solutions.


• Study properties of the NK model with Taylor rule, 
  using Dynare.
Clarida‐Gali‐Gertler Model
• Households maximize:
                               1
  E0 ∑
                              N
            logC t − exp t  t      ,  t   t−1    ,   ~iid,
                              1                        t     t
     t0


• Subject to:
           Pt C t  B t1 ≤ Wt N t  R t−1 B t  T t

• Intratemporal first order condition:
                   C t exp t N t  Wt
                                  
                                      Pt
Household Intertemporal FONC
• Condition:
                           u c,t1    Rt
                   1  E t u c,t
                                   1   t1
  – or, for when we do linearize later: 
            1  E t C t      Rt
                     C t1 1   t1
               E t explogR t  − log1   t1  − Δc t1 
              ≃  explogR t  − E t  t1 − E t Δc t1 , c t ≡ logC t 

  – take log of both sides:
  0  log  r t − E t  t1 − E t Δc t1 , r t  logR t 
  – or
        c t  − log − r t − E t  t1   c t1
Final Good Firms
• Buy                     at prices       and sell      for  Pt
      Yi,t , i ∈ 0, 1             P i,t          Yt
• Take all prices as given (competitive)
• Profits:
                                      1
                         P t Yt −  P i,t Yi,t di
                                      0

• Production function:
                                                  

                                 0 Yi,t
                                  1       −1    −1
                       Yt                 
                                                       di,   1,


• First order condition:    
                        P i,t   −                           1
                                                            1−
                                                                      1

           Yi,t  Yt                            → P t   P i,t di
                                                                     1−
                        Pt                                   0
Intermediate Good Firms
• Each ith good produced by a single monopoly 
  producer. 
• Demand curve:
                                     P i,t   −
                         Yi,t  Yt   Pt
• Technology:
                   Yi,t  expa t N i,t , Δa t  Δa t−1   a ,
                                                              t

• Calvo Price‐setting Friction
                     ̃
                     P t with probability 1 − 
         P i,t                                     ,
                     P i,t   with probability 
Marginal Cost

                                    dCost
                                                  1 − Wt /P t
real marginal cost        st     dwor ker
                                                
                                   dOutput           expa t 
                                   dwor ker


                            −1 in efficient setting
                              
                                                                   
                                  1 −        C t exp t N t
                       
                                         expa t 
The Intermediate Firm’s Decisions
• ith firm is required to satisfy whatever 
  demand shows up at its posted price.

• It’s only real decision is to adjust price 
  whenever the opportunity arises.
Intermediate Good Firm
• Present discounted value of firm profits:
                                                                    period tj profits sent to household

         marginal value of dividends to householdu c,tj /P tj     revenues            total cost
                                 
Et ∑ j                           tj                              Pi,tj Yi,tj − P tj s tj Yi,tj
  j0


               1−
• Each of the         firms that can optimize price 
           ̃
  choose      to optimize
          Pt


           in selecting price, firm only cares about
           future states in which it can’t reoptimize
   
                             
Et ∑ j                        j                                           ̃
                                                                      tj Pt Yi,tj − Ptj s tj Yi,tj .
  j0
Intermediate Good Firm Problem
• Substitute out the demand curve:
             
                E t ∑ j  tj Pt Yi,tj − P tj s tj Yi,tj 
                                   ̃
                     j0
                     
               E t ∑ j  tj Ytj P P 1− − P tj s tj P − .
                                        tj
                                            ̃t                 ̃t
                     j0
                                ̃
• Differentiate with respect to     : 
                                Pt
       
  E t ∑ j  tj Ytj P 1 − P t   Ptj s tj P−−1   0,
                          tj
                                     ̃ −                ̃t
       j0

• or
                
                                                   ̃
                                                  Pt −  s
             E t ∑     j
                                tj Ytj P 1                        0.
                                            tj
                                                  P tj  − 1 tj
               j0
Intermediate Good Firm Problem
 • Objective: 
             
                     u ′ C tj                      ̃
                                                     Pt −  s
         E t ∑ j              Ytj P1                                 0.
                         P tj          tj
                                                     Ptj  − 1 tj
             j0
                    
                                             ̃
                                            Pt −  s
            → E t ∑        j
                                   P                              0.
                                    tj
                                            P tj  − 1 tj
                   j0
 • or 
                                        
                             E t ∑ j X t,j  − p t X t,j −
                                                     ̃                s tj       0,
                                                                    −1
                                    j0




     ̃
     Pt , X 
                                    1
                          tj  tj−1  t1
                         ̄ ̄              ̄       ,j≥1
̃
pt                                                      , X t,j  X t1,j−1 1 , j  0
     Pt    t,j
                                   1, j  0.                                 t1
                                                                             ̄
Intermediate Good Firm Problem
         ̃
• Want       in:
        pt


             
       E t ∑ j0  j X t,j  − p t X t,j −
                                    ̃               
                                                   −1
                                                         s tj   0



• Solution:
                           
                    E t ∑ j0  j X t,j  −    
                                                        s
                                                   −1 tj
            ̃
            pt                                                  Kt
                        Et ∑    j0
                                     j X t,j  1−            Ft


• But, still need expressions for  Kt , Ft .

Kt  E t ∑ j X t,j  −      s tj
                                −1
          j0
                          
                                                                     −
     s t  E t ∑ j−1 1 X t1,j−1                                    s tj
    −1                       t1
                             ̄                                            −1
                          j1
                                            
                                      −
        s t  E t 1                    ∑ j X −               s
       −1             t1
                      ̄                              t1,j          − 1 t1j
                                            j0
                       E t by LIME                       
                                                   −
        s t   E t E                     1           ∑ j X −              s
       −1               t1
                                            t1
                                           ̄                       t1,j
                                                                                 − 1 t1j
                                                         j0
                                                              exactly K t1 !
                                                   
                                      −
        s t  E t 1                     E t1 ∑ j X −                 s
       −1             t1
                      ̄                                     t1,j
                                                                             − 1 t1j
                                                   j0
                                      −
     s t  E t 1                       Kt1
    −1             t1
                   ̄
• From previous slide:
                                               −
           Kt   s t  E t 1                     Kt1 .
               −1             t1
                              ̄

• Substituting out for marginal cost:
                               dCost/dlabor

         s t   1 −  Wt /P t
       −1     −1       dOutput/dlabor

                               expa t 

                                   Wt
                                  Pt
                                        by household optimization

                                                       
                    1 −             exp t N t C t
                                                                   .
                  −1                     expa t 
In Sum
• solution:

                         
                 E t ∑ j0  j X t,j  −    
                                                     s
                                                −1 tj
         ̃
         pt                                              Kt ,
                      E t ∑ j0  j X t,j   1−        Ft

• Where:
                                         
                      exp t N t C t                              −
   Kt  1 −  t                      E t 1                          Kt1 .
                  −1   expa t                t1
                                               ̄

                
                                                                  1−
     F t ≡ E t ∑ X t,j 
                          j       1−
                                         1  E t         1           F t1
                                                           t1
                                                          ̄
                j0
To Characterize Equilibrium
• Have equations characterizing optimization by 
  firms and households.
• Still need:
                                               P i,t , 0 ≤ i ≤ 1
  – Expression for all the prices. Prices,                          , 
    will all be different because of the price setting 
    frictions.
  – Relationship between aggregate employment and 
    aggregate output not simple because of price 
    distortions:
                  Y t ≠ e a t N t , in general
Going for Prices
 • Aggregate price relationship                                             Calvo insight:
                                                                            This is just a simple
                             1
                                                                            function of last period’s
        0 P 1− di
          1                 1−
Pt          i,t                                                            aggregate price because
                                                                            non-optimizers chosen at
                                                                            random.
                                                                                                        1

        firms that reoptimize price P 1− di  firms that don’t reoptimize price P 1− di
                                                                                                       1−
                                      i,t                                             i,t



       all reoptimizers choose same price                                                                                  1
                    
                                                  ̃ 1−  
                                            1 − P t                                                 P i,t
                                                                                                             1−
                                                                                                                     di
                                                                                                                          1−

                                                                   firms that don’t reoptimize price



 • In principle, to solve the model need all the 
   prices, P t , P i,t , 0 ≤ i ≤ 1
       – Fortunately, that won’t be necessary. 
̃
Expression for     in terms of aggregate 
               pt
               inflation 
 • Conclude that this relationship holds between 
   prices:                                                 1
              P t  1 −       ̃ 1−
                             P t       
                                                1−
                                             P t−1       1−
                                                                .

    – Only two variables here!
 • Divide by      :
              Pt
                                                                 1
                               1−                  1−
              1             ̃
                      1 − p t            1                 1−

                                              t
                                              ̄
 • Rearrange:
                                                 1
                                      −1
                             1−     t
                                    ̄           1−
                      ̃
                      pt 
                               1−
Relation Between Aggregate 
    Output and Aggregate Inputs
• Technically, there is no ‘aggregate production 
  function’ in this model
  – If you know how many people are working, N, and 
    the state of technology, a, you don’t have enough 
    information to know what Y is.
  – Price frictions imply that resources will not be 
    efficiently allocated among different inputs.
     • Implies Y low for given a and N. How low?
     • Tak Yun (JME) gave a simple answer.
Tak Yun Algebra
                                                   labor market clearing
     Y∗    0 Yi,t di
             1
                                 0 At N i,t di
                                   1
                                                             
                                                                          At N t
      t




             demand curve                          −
                 
                           Yt 
                                    1   P i,t
                                                        di
                                    0   Pt


                     0 P i,t  −di
                       1
           Yt P 
                 t

                                                                     Calvo insight

        Y t P  P ∗  −
               t    t


                                            −1
• Where:         P∗
                  t    ≡    0
                              1
                                  P − di
                                    i,t
                                            
                                                             ̃t
                                                    1 − P −  P ∗  − 
                                                                        t−1
                                                                                     −1
                                                                                     
Relationship Between Agg Inputs 
         and Agg Output
• Rewriting previous equation:
                                          
                                  P∗
                        Yt        t
                                              Y∗
                                               t
                                  Pt


                             p ∗ e at N t ,
                                t

• ‘efficiency distortion’:

                      ≤1
             p∗
              t   :
                       1 P i,t  P j,t , all i, j
Collecting Equilibrium Conditions
• Price setting:
                                    
        K t  1 −    exp t N t C t  E t   K (1)
                                                  ̄ t1 t1
                      −1     At

                      F t  1  E t  −1 F t1 (2)
                                      ̄ t1

• Intermediate good firm optimality and 
  restriction across prices:
                                        ̃
                                       p t by restriction across prices
              ̃
             p t by firm optimality
                                                    −1
                                                                    1

                    Kt                     1−      t
                                                   ̄               1−
                                                                          (3)
                    Ft                       1−
Equilibrium Conditions
• Law of motion of (Tak Yun) distortion:
                                                          −1
                                −1
                         1−   t
                              ̄         −1
                                                    
                                                     ̄t
      p∗      1 −                               ∗          (4)
       t
                           1−                     p t−1

• Household Intertemporal Condition:
                   1  E t 1 R t (5)
                   Ct      C t1  t1
                                 ̄
• Aggregate inputs and output:
                     C t  p ∗ e a t N t (6)
                             t

• 6 equations, 8 unknowns:
             , C t , p ∗ , N t ,  t , K t , F t , R t
                        t         ̄
• System under determined!
Underdetermined System
• Not surprising: we added a variable, the 
  nominal rate of interest.

• Also, we’re counting subsidy as among the 
  unknowns.

• Have two extra policy variables.

• One way to pin them down: compute optimal 
  policy.
Ramsey‐Optimal Policy
• 6 equations in 8 unknowns…..
  – Many configurations of the 8 unknowns that 
    satisfy the 6 equations.
  – Look for the best configurations (Ramsey optimal)
     • Value of tax subsidy and of R represent optimal policy


• Finding the Ramsey optimal setting of the 6 
  variables involves solving a simple Lagrangian 
  optimization problem.
Ramsey Problem
                           
                                                                 N 1
            max                    E 0 ∑  t  logC t − exp t  t
,p ∗ ,C t ,N t ,Rt , t ,F t ,K t
    t                ̄                                           1
                          t0




  1t    1 − Et  Rt
          Ct    C t1  t1
                      ̄
                                                         
                                                −1
           1 −               1 −  t 
                                   ̄                    −1
                                                                 
                                                                 ̄
  2t               1 −                                    ∗t
           p∗
            t                   1−                            p t−1
  3t 1  E t  −1 F t1 − F t 
                ̄ t1
                    C t exp t N 
  4t   1 −                   t
                                      E t   K t1 − K t
                                             ̄ t1
                −1       e at

                                   1
                1−     −1
                       ̄t         1−
  5t F t                              − Kt
                 1−
  6t C t − p ∗ e a t N t 
                t
Solving the Ramsey Problem 
            (surprisingly easy in this case)
   • First, substitute out consumption everywhere
                                     
                                                                                   N 1
                      max                   E 0 ∑  t  logN t  logp ∗ − exp t  t
              ,p ∗ ,N t ,Rt , t ,F t ,K t
                  t           ̄
                                                                      t
                                                                                   1
                                    t0

 defines R               1 − Et       e at    Rt
                1t    ∗
                       pt Nt    p t1 e N t1  t1
                                  ∗    a t1  ̄
                                                                   
                                                           −1
                        1 −               1 −  t 
                                                ̄                 −1
                                                                           
                                                                           ̄
                2t              1 −                                 ∗t
                        p∗
                         t                   1−                         p t−1
defines F
                              ̄ −1
                3t 1  E t  t1 F t1 − F t 
defines tax     4t 1 −   exp t N 1 p ∗  E t   K t1 − K t
                                                          ̄ t1
                                 −1                t t
                                              1
                             1−    −1
                                   ̄t        1−
                5t F t                           − Kt 
 defines K                    1−
Solving the Ramsey Problem, cnt’d 
• Simplified problem:
                                 
                                                                           N 1
                 max E 0 ∑   logN t t
                                                       logp ∗   − exp t  t
                 t ,p ∗ ,N t
                ̄ t
                                                            t
                                                                           1
                                 t0


                                                                        
                                                                 −1
                                1 −                1 −  t 
                                                         ̄             −1
                                                                                
                                                                                ̄
                  2t                     1 −                             ∗t         
                                p∗
                                 t                    1−                     p t−1

• First order conditions with respect to p ∗ ,  t , N t
                                           t ̄
                                                                         1
                                                     p ∗  −1         −1
  p ∗   2,t1     2t ,  t 
                  ̄ t1        ̄                        t−1
                                                                              , N t  exp −  t
    t
                                                1−     p ∗  −1
                                                             t−1
                                                                                           1


• Substituting the solution for inflation into law 
  of motion for price distortion: 
                                                                                1
                                 p∗
                                  t     1 −          p ∗  −1
                                                              t−1
                                                                              −1
                                                                                      .
Solution to Ramsey Problem
Eventually, price distortions
eliminated, regardless of shocks
                                                                              1
                                p ∗  1 −   p t−1  −1
                                  t
                                                    ∗                       −1


  When price distortions              p∗
  gone, so is inflation.         t  t−1
                                ̄
                                      p∗t

                              N t  exp −          t
  Efficient (‘first best’)                     1
  allocations in real
  economy                    1−  −1  
                              C t  p ∗ e at N t .
                                      t
      Consumption corresponds to efficient
      allocations in real economy, eventually when price distortions gone
Eventually, Optimal (Ramsey) Equilibrium and 
                Efficient Allocations in Real Economy Coincide
                                        Convergence of price distortion


         0.98



         0.96



         0.94


                                           0. 75,   10
         0.92
p-star




          0.9

                                                                            1
         0.88                  p∗
                                t    1 −           p ∗  −1
                                                            t−1
                                                                          −1



         0.86



         0.84



         0.82



          0.8
                2    4     6        8              10           12                14   16   18   20
• The Ramsey allocations are eventually the 
  best allocations in the economy without price 
  frictions (i.e., ‘first best allocations’)

• Refer to the Ramsey allocations as the ‘natural 
  allocations’….
  – Natural consumption, natural rate of interest, etc.
Equations of the NK Model Under the 
Optimal Policy (‘Natural Equilibrium’)
• Output and employment is (eventually)

      y∗  at −    1 t , n∗  − 1 t
       t
                  1      t
                                1
• Intertemporal Euler equation after taking logs 
  and ignoring variance adjustment term:
  y ∗  −r ∗ − E t  ∗ − rr  E t y ∗ , rr  − log
    t       t         t1             t1


• Inflation in Ramsey equilibrium is (eventually) 
  zero.
Solving for Natural Rate of Interest
• Intertemporal euler equation in natural 
  equilibrium:
        ∗                           y∗
          yt                                            t1


   at −     1
           1
                  t  −r ∗ − rr  E t
                           t                 a t1 −     1
                                                        1
                                                               t1
• Back out the natural rate:

               r ∗  rr  Δa t 
                 t
                                     1
                                    1
                                           1 −  t
• Shocks:
            t   t−1    , Δa t  Δa t−1   t
                            t
Next, Put Turn to the NK Model 
       with Taylor Rule
Taylor Rule

• Taylor rule: designed, so that in steady state, 
                      1
                       ̄
  inflation is zero (             )

• Employment subsidy extinguishes monopoly 
  power in steady state: 
                 1 −    1
                        −1
NK IS Curve
  • Euler equation in two equilibria:

Taylor rule equilibrium: y t  −r t − E t  t1 − rr  E t y t1


    Natural equilibrium: y ∗  −r ∗ − rr  E t y ∗
                           t       t               t1



  • Subtract:                                              Output gap



             x t  −r t − E t  t1 − r ∗   E t x t1
                                         t
Output in NK Equilibrium
• Agg output relation:

                                            0 if P i,t  P j,t for all i, j
yt    logp ∗
            t    nt  at ,   logp ∗
                                   t                                          .
                                           ≤0           otherwise

• To first order approximation, 

                ̂t     ̂ t−1
                p ∗ ≈ p ∗  0   t , → p ∗ ≈ 1
                                 ̄          t
Price Setting Equations
• Log‐linearly expand the price setting equations 
  about steady state.                     1
                                                   1− −1
                                                      ̄t
                                                                      − Kt  0
                                                                1−
1       ̄ −1
     E t  t1 F t1    − Ft  0           Ft      1−
                                  
                  C t exp t N t
     1 −     −1      eat
                                              ̄
                                       E t  t1 K t1 − K t  0

• Log‐linearly expand about steady state:
                       1−1−                  
              t 
              ̄                       1  x t   t1
                                                     ̄

• See http://faculty.wcas.northwestern.edu/~lchrist/course/solving_handout.pdf
Taylor Rule
 • Policy rule



r t  r t−1  1 − rr     t   x x t   u t , , x t ≡ y t − y ∗
                                                                        t
Equations of Actual Equilibrium
          Closed by Adding Policy Rule 

                       E t  t1  x t −  t  0 (Phillips curve)


      − r t − E t  t1 − r ∗   E t x t1 − x t  0 (IS equation)
                             t



r t−1  1 −    t  1 −  x x t − r t  0 (policy rule)


           r ∗ − Δa t −    1 1 −  t  0 (definition of natural rate)
             t
                           1
Solving the Model
                  Δa t                    0                Δa t−1                    t
        st                                                                  
                   t                    0                    t−1                   
                                                                                       t

        s t  Ps t−1   t
     0 0 0              t1                 −1                          0      0            t
    1
       1 0 0           x t1                  0              −1         −
                                                                          1       1
                                                                                              xt
                                    
    0 0 0 0             r t1            1 −   1 −  x            −1      0            rt
    0 0 0 0             r∗
                         t1                   0               0           0      1            rr ∗
                                                                                                  t


    0 0 0 0           t−1               0 0 0                            0                0
    0 0 0 0             x t−1            0 0 0                            0                0
                                                       s t1                                       st
    0 0  0             r t−1            0 0 0                            0                0
    0 0 0 0             r∗
                         t−1             0 0 0                        − −  1 − 
                                                                              1




                   E t  0 z t1   1 z t   2 z t−1   0 s t1   1 s t   0
Solving the Model
      E t  0 z t1   1 z t   2 z t−1   0 s t1   1 s t   0


                                            s t − Ps t−1 −  t  0.

• Solution:
                          z t  Az t−1  Bs t

• As before:
                         0 A2   1 A   2 I  0,


           F   0   0 BP   1   0 A   1 B  0
 x  0,    1. 5,   0. 99,   1,   0. 2,   0. 75,   0,   0. 2,   0. 5.

                            Dynamic Response to a Technology Shock
                inflation                                 output gap                    nominal rate
                                                                              0.2
                                         0.15
 0.03                                                                        0.15            natural nominal rate
                                          0.1                                                actual nominal rate
 0.02                                                                         0.1

 0.01                                    0.05                                0.05


        0    2         4        6               0      2       4         6          0   2       4        6
            natural real rate                        log, technology                        output
  0.2

 0.15                                       1                                 1.2
                                                                                                 natural output
                                                                             1.15
  0.1                                                                                            actual output
                                          0.5                                 1.1
 0.05                                                                        1.05
                                            0                                   1
        0    2     4            6               0     2         4        6          0   2       4        6
             employment
  0.2

 0.15

  0.1
                                                    natural employment
 0.05                                               actual employment

    0
        0           5               10
Dynamic Response to a Preference Shock
                inflation                           output gap                        nominal rate
 0.1                                                                       0.25
0.08                                0.25                                    0.2
                                     0.2                                                           natural real rate
0.06                                                                       0.15
                                    0.15                                                           actual nominal rate
0.04                                                                        0.1
                                     0.1
0.02                                0.05                                   0.05

       0    2         4        6           0     2       4      6                 0   2       4          6
           natural real rate                   preference shock                           output
0.25                                   1
 0.2                                                                       -0.1
0.15                                                                       -0.2
                                     0.5
 0.1                                                                       -0.3                    natural output
                                                                                                   actual output
0.05                                                                       -0.4
                                       0                                   -0.5
       0    2     4            6           0    2         4       6               0   2       4          6
            employment


-0.1
-0.2                                                  natural employment
                                                      actual employment
-0.3
-0.4
-0.5
       0    2         4        6
Conclusion of NK Model Analysis
• We studied examples in which the Taylor rule 
  moves the interest rate in the right direction 
  in response to shocks.

• However, the move is not strong enough. Will 
  consider modifications of the Taylor rule using 
  Dynare.

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Lecture on nk [compatibility mode]

  • 1. Simple New Keynesian Model  without Capital Lawrence J. Christiano
  • 2. Outline • Formulate the nonlinear equilibrium conditions of the  model. – Need actual nonlinear conditions to study Ramsey‐optimal  policy, even if we want to use linearization methods to  study Ramsey.  • Ramsey will be used to define ‘output gap’ in positive model of the  economy, in which monetary policy is governed by the Taylor rule. • Later, when discussing ‘timeless perspective’, will discuss use of  Ramsey‐optimal policy in actual, real‐time implementation of  monetary policy.   – Need nonlinear equations if we were to study higher order  perturbation solutions. • Study properties of the NK model with Taylor rule,  using Dynare.
  • 3. Clarida‐Gali‐Gertler Model • Households maximize:  1 E0 ∑ N logC t − exp t  t ,  t   t−1    ,   ~iid, 1 t t t0 • Subject to: Pt C t  B t1 ≤ Wt N t  R t−1 B t  T t • Intratemporal first order condition: C t exp t N t  Wt  Pt
  • 4. Household Intertemporal FONC • Condition: u c,t1 Rt 1  E t u c,t 1   t1 – or, for when we do linearize later:  1  E t C t Rt C t1 1   t1  E t explogR t  − log1   t1  − Δc t1  ≃  explogR t  − E t  t1 − E t Δc t1 , c t ≡ logC t  – take log of both sides: 0  log  r t − E t  t1 − E t Δc t1 , r t  logR t  – or c t  − log − r t − E t  t1   c t1
  • 5. Final Good Firms • Buy                     at prices       and sell      for  Pt Yi,t , i ∈ 0, 1 P i,t Yt • Take all prices as given (competitive) • Profits: 1 P t Yt −  P i,t Yi,t di 0 • Production function:   0 Yi,t 1 −1 −1 Yt   di,   1, • First order condition:     P i,t − 1 1− 1 Yi,t  Yt → P t   P i,t di 1− Pt 0
  • 6. Intermediate Good Firms • Each ith good produced by a single monopoly  producer.  • Demand curve: P i,t − Yi,t  Yt Pt • Technology: Yi,t  expa t N i,t , Δa t  Δa t−1   a , t • Calvo Price‐setting Friction ̃ P t with probability 1 −  P i,t  , P i,t with probability 
  • 7. Marginal Cost dCost 1 − Wt /P t real marginal cost  st  dwor ker  dOutput expa t  dwor ker  −1 in efficient setting   1 −  C t exp t N t  expa t 
  • 8. The Intermediate Firm’s Decisions • ith firm is required to satisfy whatever  demand shows up at its posted price. • It’s only real decision is to adjust price  whenever the opportunity arises.
  • 9. Intermediate Good Firm • Present discounted value of firm profits: period tj profits sent to household  marginal value of dividends to householdu c,tj /P tj revenues total cost  Et ∑ j  tj Pi,tj Yi,tj − P tj s tj Yi,tj j0 1− • Each of the         firms that can optimize price  ̃ choose      to optimize Pt in selecting price, firm only cares about future states in which it can’t reoptimize   Et ∑ j j ̃  tj Pt Yi,tj − Ptj s tj Yi,tj . j0
  • 10. Intermediate Good Firm Problem • Substitute out the demand curve:  E t ∑ j  tj Pt Yi,tj − P tj s tj Yi,tj  ̃ j0   E t ∑ j  tj Ytj P P 1− − P tj s tj P − . tj ̃t ̃t j0 ̃ • Differentiate with respect to     :  Pt  E t ∑ j  tj Ytj P 1 − P t   Ptj s tj P−−1   0, tj ̃ − ̃t j0 • or  ̃ Pt −  s E t ∑ j  tj Ytj P 1  0. tj P tj  − 1 tj j0
  • 11. Intermediate Good Firm Problem • Objective:   u ′ C tj  ̃ Pt −  s E t ∑ j Ytj P1  0. P tj tj Ptj  − 1 tj j0  ̃ Pt −  s → E t ∑ j P  0. tj P tj  − 1 tj j0 • or   E t ∑ j X t,j  − p t X t,j − ̃  s tj  0, −1 j0 ̃ Pt , X  1  tj  tj−1  t1 ̄ ̄ ̄ ,j≥1 ̃ pt  , X t,j  X t1,j−1 1 , j  0 Pt t,j 1, j  0.  t1 ̄
  • 12. Intermediate Good Firm Problem ̃ • Want       in: pt  E t ∑ j0  j X t,j  − p t X t,j − ̃  −1 s tj 0 • Solution:  E t ∑ j0  j X t,j  −  s −1 tj ̃ pt    Kt Et ∑ j0  j X t,j  1− Ft • But, still need expressions for  Kt , Ft .
  • 13.  Kt  E t ∑ j X t,j  −  s tj −1 j0  −   s t  E t ∑ j−1 1 X t1,j−1  s tj −1  t1 ̄ −1 j1  −   s t  E t 1 ∑ j X −  s −1  t1 ̄ t1,j  − 1 t1j j0 E t by LIME  −   s t   E t E 1 ∑ j X −  s −1 t1  t1 ̄ t1,j  − 1 t1j j0 exactly K t1 !  −   s t  E t 1 E t1 ∑ j X −  s −1  t1 ̄ t1,j  − 1 t1j j0 −   s t  E t 1 Kt1 −1  t1 ̄
  • 14. • From previous slide: − Kt   s t  E t 1 Kt1 . −1  t1 ̄ • Substituting out for marginal cost: dCost/dlabor  s t   1 −  Wt /P t −1 −1 dOutput/dlabor expa t  Wt  Pt by household optimization   1 −  exp t N t C t  . −1 expa t 
  • 15. In Sum • solution:  E t ∑ j0  j X t,j  −  s −1 tj ̃ pt    Kt , E t ∑ j0  j X t,j  1− Ft • Where:  exp t N t C t − Kt  1 −  t    E t 1 Kt1 . −1 expa t   t1 ̄  1− F t ≡ E t ∑ X t,j  j 1−  1  E t 1 F t1  t1 ̄ j0
  • 16. To Characterize Equilibrium • Have equations characterizing optimization by  firms and households. • Still need: P i,t , 0 ≤ i ≤ 1 – Expression for all the prices. Prices,                          ,  will all be different because of the price setting  frictions. – Relationship between aggregate employment and  aggregate output not simple because of price  distortions: Y t ≠ e a t N t , in general
  • 17. Going for Prices • Aggregate price relationship Calvo insight: This is just a simple 1 function of last period’s 0 P 1− di 1 1− Pt  i,t aggregate price because non-optimizers chosen at random. 1 firms that reoptimize price P 1− di  firms that don’t reoptimize price P 1− di 1−  i,t i,t all reoptimizers choose same price 1   ̃ 1−   1 − P t P i,t 1− di 1− firms that don’t reoptimize price • In principle, to solve the model need all the  prices, P t , P i,t , 0 ≤ i ≤ 1 – Fortunately, that won’t be necessary. 
  • 18. ̃ Expression for     in terms of aggregate  pt inflation  • Conclude that this relationship holds between  prices: 1 P t  1 − ̃ 1− P t  1− P t−1 1− . – Only two variables here! • Divide by      : Pt 1 1− 1− 1 ̃ 1 − p t  1 1− t ̄ • Rearrange: 1 −1 1−  t ̄ 1− ̃ pt  1−
  • 19. Relation Between Aggregate  Output and Aggregate Inputs • Technically, there is no ‘aggregate production  function’ in this model – If you know how many people are working, N, and  the state of technology, a, you don’t have enough  information to know what Y is. – Price frictions imply that resources will not be  efficiently allocated among different inputs. • Implies Y low for given a and N. How low? • Tak Yun (JME) gave a simple answer.
  • 20. Tak Yun Algebra labor market clearing Y∗  0 Yi,t di 1  0 At N i,t di 1   At N t t demand curve −   Yt  1 P i,t di 0 Pt 0 P i,t  −di 1  Yt P  t Calvo insight  Y t P  P ∗  − t t −1 • Where: P∗ t ≡ 0 1 P − di i,t  ̃t  1 − P −  P ∗  −  t−1 −1 
  • 21. Relationship Between Agg Inputs  and Agg Output • Rewriting previous equation:  P∗ Yt  t Y∗ t Pt  p ∗ e at N t , t • ‘efficiency distortion’: ≤1 p∗ t :  1 P i,t  P j,t , all i, j
  • 22. Collecting Equilibrium Conditions • Price setting:  K t  1 −   exp t N t C t  E t   K (1) ̄ t1 t1 −1 At F t  1  E t  −1 F t1 (2) ̄ t1 • Intermediate good firm optimality and  restriction across prices: ̃ p t by restriction across prices ̃ p t by firm optimality  −1 1 Kt 1−  t ̄ 1−  (3) Ft 1−
  • 23. Equilibrium Conditions • Law of motion of (Tak Yun) distortion:  −1 −1 1−  t ̄ −1   ̄t p∗  1 −   ∗ (4) t 1− p t−1 • Household Intertemporal Condition: 1  E t 1 R t (5) Ct C t1  t1 ̄ • Aggregate inputs and output: C t  p ∗ e a t N t (6) t • 6 equations, 8 unknowns: , C t , p ∗ , N t ,  t , K t , F t , R t t ̄ • System under determined!
  • 24. Underdetermined System • Not surprising: we added a variable, the  nominal rate of interest. • Also, we’re counting subsidy as among the  unknowns. • Have two extra policy variables. • One way to pin them down: compute optimal  policy.
  • 25. Ramsey‐Optimal Policy • 6 equations in 8 unknowns….. – Many configurations of the 8 unknowns that  satisfy the 6 equations. – Look for the best configurations (Ramsey optimal) • Value of tax subsidy and of R represent optimal policy • Finding the Ramsey optimal setting of the 6  variables involves solving a simple Lagrangian  optimization problem.
  • 26. Ramsey Problem  N 1 max E 0 ∑  t  logC t − exp t  t ,p ∗ ,C t ,N t ,Rt , t ,F t ,K t t ̄ 1 t0   1t 1 − Et  Rt Ct C t1  t1 ̄  −1 1 − 1 −  t  ̄ −1   ̄   2t 1 −   ∗t p∗ t 1− p t−1   3t 1  E t  −1 F t1 − F t  ̄ t1 C t exp t N    4t 1 −   t  E t   K t1 − K t ̄ t1 −1 e at 1 1−  −1 ̄t 1−   5t F t − Kt 1−   6t C t − p ∗ e a t N t  t
  • 27. Solving the Ramsey Problem  (surprisingly easy in this case) • First, substitute out consumption everywhere  N 1 max E 0 ∑  t  logN t  logp ∗ − exp t  t ,p ∗ ,N t ,Rt , t ,F t ,K t t ̄ t 1 t0 defines R 1 − Et e at  Rt   1t ∗ pt Nt p t1 e N t1  t1 ∗ a t1 ̄  −1 1 − 1 −  t  ̄ −1   ̄   2t 1 −   ∗t p∗ t 1− p t−1 defines F ̄ −1   3t 1  E t  t1 F t1 − F t  defines tax   4t 1 −   exp t N 1 p ∗  E t   K t1 − K t ̄ t1 −1 t t 1 1−  −1 ̄t 1−   5t F t − Kt  defines K 1−
  • 28. Solving the Ramsey Problem, cnt’d  • Simplified problem:  N 1 max E 0 ∑   logN t t logp ∗ − exp t  t  t ,p ∗ ,N t ̄ t t 1 t0  −1 1 − 1 −  t  ̄ −1   ̄   2t 1 −   ∗t  p∗ t 1− p t−1 • First order conditions with respect to p ∗ ,  t , N t t ̄ 1 p ∗  −1 −1 p ∗   2,t1     2t ,  t  ̄ t1 ̄ t−1 , N t  exp −  t t 1−   p ∗  −1 t−1 1 • Substituting the solution for inflation into law  of motion for price distortion:  1 p∗ t  1 −   p ∗  −1 t−1 −1 .
  • 29. Solution to Ramsey Problem Eventually, price distortions eliminated, regardless of shocks 1 p ∗  1 −   p t−1  −1 t ∗ −1 When price distortions p∗ gone, so is inflation.  t  t−1 ̄ p∗t N t  exp − t Efficient (‘first best’) 1 allocations in real economy 1−  −1  C t  p ∗ e at N t . t Consumption corresponds to efficient allocations in real economy, eventually when price distortions gone
  • 30. Eventually, Optimal (Ramsey) Equilibrium and  Efficient Allocations in Real Economy Coincide Convergence of price distortion 0.98 0.96 0.94   0. 75,   10 0.92 p-star 0.9 1 0.88 p∗ t  1 −   p ∗  −1 t−1 −1 0.86 0.84 0.82 0.8 2 4 6 8 10 12 14 16 18 20
  • 31. • The Ramsey allocations are eventually the  best allocations in the economy without price  frictions (i.e., ‘first best allocations’) • Refer to the Ramsey allocations as the ‘natural  allocations’…. – Natural consumption, natural rate of interest, etc.
  • 32. Equations of the NK Model Under the  Optimal Policy (‘Natural Equilibrium’) • Output and employment is (eventually) y∗  at − 1 t , n∗  − 1 t t 1 t 1 • Intertemporal Euler equation after taking logs  and ignoring variance adjustment term: y ∗  −r ∗ − E t  ∗ − rr  E t y ∗ , rr  − log t t t1 t1 • Inflation in Ramsey equilibrium is (eventually)  zero.
  • 33. Solving for Natural Rate of Interest • Intertemporal euler equation in natural  equilibrium: ∗ y∗ yt t1 at − 1 1  t  −r ∗ − rr  E t t a t1 − 1 1  t1 • Back out the natural rate: r ∗  rr  Δa t  t 1 1 1 −  t • Shocks:  t   t−1    , Δa t  Δa t−1   t t
  • 35. Taylor Rule • Taylor rule: designed, so that in steady state,  1 ̄ inflation is zero (             ) • Employment subsidy extinguishes monopoly  power in steady state:  1 −    1 −1
  • 36. NK IS Curve • Euler equation in two equilibria: Taylor rule equilibrium: y t  −r t − E t  t1 − rr  E t y t1 Natural equilibrium: y ∗  −r ∗ − rr  E t y ∗ t t t1 • Subtract: Output gap x t  −r t − E t  t1 − r ∗   E t x t1 t
  • 37. Output in NK Equilibrium • Agg output relation:  0 if P i,t  P j,t for all i, j yt  logp ∗ t  nt  at , logp ∗ t  . ≤0 otherwise • To first order approximation,  ̂t ̂ t−1 p ∗ ≈ p ∗  0   t , → p ∗ ≈ 1 ̄ t
  • 38. Price Setting Equations • Log‐linearly expand the price setting equations  about steady state. 1 1− −1 ̄t − Kt  0 1− 1 ̄ −1 E t  t1 F t1 − Ft  0 Ft 1−   C t exp t N t 1 −  −1 eat ̄  E t  t1 K t1 − K t  0 • Log‐linearly expand about steady state:  1−1−  t  ̄  1  x t   t1 ̄ • See http://faculty.wcas.northwestern.edu/~lchrist/course/solving_handout.pdf
  • 39. Taylor Rule • Policy rule r t  r t−1  1 − rr     t   x x t   u t , , x t ≡ y t − y ∗ t
  • 40. Equations of Actual Equilibrium Closed by Adding Policy Rule  E t  t1  x t −  t  0 (Phillips curve) − r t − E t  t1 − r ∗   E t x t1 − x t  0 (IS equation) t r t−1  1 −    t  1 −  x x t − r t  0 (policy rule) r ∗ − Δa t − 1 1 −  t  0 (definition of natural rate) t 1
  • 41. Solving the Model Δa t  0 Δa t−1 t st    t 0   t−1  t s t  Ps t−1   t  0 0 0  t1 −1  0 0 t 1  1 0 0 x t1 0 −1 − 1 1  xt  0 0 0 0 r t1 1 −   1 −  x −1 0 rt 0 0 0 0 r∗ t1 0 0 0 1 rr ∗ t 0 0 0 0  t−1 0 0 0 0 0 0 0 0 0 x t−1 0 0 0 0 0   s t1  st 0 0  0 r t−1 0 0 0 0 0 0 0 0 0 r∗ t−1 0 0 0 − −  1 −  1 E t  0 z t1   1 z t   2 z t−1   0 s t1   1 s t   0
  • 42. Solving the Model E t  0 z t1   1 z t   2 z t−1   0 s t1   1 s t   0 s t − Ps t−1 −  t  0. • Solution: z t  Az t−1  Bs t • As before:  0 A2   1 A   2 I  0, F   0   0 BP   1   0 A   1 B  0
  • 43.  x  0,    1. 5,   0. 99,   1,   0. 2,   0. 75,   0,   0. 2,   0. 5. Dynamic Response to a Technology Shock inflation output gap nominal rate 0.2 0.15 0.03 0.15 natural nominal rate 0.1 actual nominal rate 0.02 0.1 0.01 0.05 0.05 0 2 4 6 0 2 4 6 0 2 4 6 natural real rate log, technology output 0.2 0.15 1 1.2 natural output 1.15 0.1 actual output 0.5 1.1 0.05 1.05 0 1 0 2 4 6 0 2 4 6 0 2 4 6 employment 0.2 0.15 0.1 natural employment 0.05 actual employment 0 0 5 10
  • 44. Dynamic Response to a Preference Shock inflation output gap nominal rate 0.1 0.25 0.08 0.25 0.2 0.2 natural real rate 0.06 0.15 0.15 actual nominal rate 0.04 0.1 0.1 0.02 0.05 0.05 0 2 4 6 0 2 4 6 0 2 4 6 natural real rate preference shock output 0.25 1 0.2 -0.1 0.15 -0.2 0.5 0.1 -0.3 natural output actual output 0.05 -0.4 0 -0.5 0 2 4 6 0 2 4 6 0 2 4 6 employment -0.1 -0.2 natural employment actual employment -0.3 -0.4 -0.5 0 2 4 6
  • 45. Conclusion of NK Model Analysis • We studied examples in which the Taylor rule  moves the interest rate in the right direction  in response to shocks. • However, the move is not strong enough. Will  consider modifications of the Taylor rule using  Dynare.