concept-learning.ppt

Waiting outside the house to get an autograph.
2

Which days does he come out to enjoy sports?
• Sky condition
• Humidity
• Temperature
• Wind
• Water
• Forecast
• Attributes of a day: takes on values
3

Learning Task
• We want to make a hypothesis about the day on which
SRK comes out..
– in the form of a boolean function on the attributes of the day.
• Find the right hypothesis/function from historical data
4

Training Examples for EnjoySport
c( )=1
c( )=1
c( )=1
c( )=0
 Negative and positive learning examples
 Concept learning:
- Deriving a Boolean function from training examples
- Many “hypothetical” boolean functions
 Hypotheses; find h such that h = c.
– Other more complex examples:
 Non-boolean functions
 Generate hypotheses for concept from TE’s
5
c is the target concept

Representing Hypotheses
• Task of finding appropriate set of hypotheses for concept given training data
• Represent hypothesis as Conjunction of constraints of the following form:
– Values possible in any hypothesis
 Specific value : Water = Warm
 Don’t-care value: Water = ?
 No value allowed : Water = 
– i.e., no permissible value given values of other attributes
– Use vector of such values as hypothesis:
  Sky AirTemp Humid Wind Water Forecast 
– Example: Sunny ? ? Strong ? Same 
• Idea of satisfaction of hypothesis by some example
– say “example satisfies hypothesis”
– defined by a function h(x):
h(x) = 1 if h is true on x
= 0 otherwise
• Want hypothesis that best fits examples:
– Can reduce learning to search problem over space of hypotheses
6

Prototypical Concept Learning Task
TASK T: predicting when person will enjoy sport
– Target function c: EnjoySport : X  {0, 1}
– Cannot, in general, know Target function c
 Adopt hypotheses H about c
– Form of hypotheses H:
 Conjunctions of literals ?, Cold, High, ?, ?, ? 
 EXPERIENCE E
– Instances X: possible days described by attributes Sky, AirTemp, Humidity,
Wind, Water, Forecast
– Training examples D: Positive/negative examples of target function {x1,
c(x1), . . . xm, c(xm)}
 PERFORMANCE MEASURE P: Hypotheses h in H such that h(x) = c(x) for all x
in D ()
– There may exist several alternative hypotheses that fit examples
7

Inductive Learning Hypothesis
Any hypothesis found to approximate the target
function well over a sufficiently large set of
training examples will also approximate the target
function well over other unobserved examples
8

Approaches to learning algorithms
• Brute force search
– Enumerate all possible hypotheses and evaluate
– Highly inefficient even for small EnjoySport example
 |X| = 3.2.2.2.2= 96 distinct instances
 Large number of syntactically distinct hypotheses (0’s, ?’s)
– EnjoySport: |H| = 5.4.4.4.4.4=5120
– Fewer when consider h’s with 0’s
Every h with a 0 is empty set of instances (classifies instance as neg)
Hence # semantically distinct h’s is:
1+ (4.3.3.3.3.3) = 973
 EnjoySport is VERY small problem compared to many
• Hence use other search procedures.
– Approach 1: Search based on ordering of hypotheses
– Approach 2: Search based on finding all possible hypotheses using
a good representation of hypothesis space
 All hypotheses that fit data
9
The choice of the hypothesis
space reduces the number of
hypotheses.

Ordering on Hypotheses
Instances X Hypotheses H
specific
general
h is more general than h( h g h) if for each instance x,
h(x) = 1  h(x) = 1
 Which is the most general/most specific hypothesis?
x1=Sunny Warm High Strong Cool Same
x2=Sunny Warm High Light Warm Same
h1=Sunny ? ? Strong ? ?
h2=Sunny ? ? ? ? ?
h3=Sunny ? ? ? Cool ?

Find-S Algorithm
Assumes
There is hypothesis h in H describing target function c
There are no errors in the TEs
Procedure
1. Initialize h to the most specific hypothesis in H (what is this?)
2. For each positive training instance x
For each attribute constraint ai in h
If the constraint ai in h is satisfied by x
do nothing
Else
replace ai in h by the next more general constraint that is satisfied by x
3. Output hypothesis h
Note
There is no change for a negative example, so they are ignored.
This follows from assumptions that there is h in H describing target function c (ie.,for this h, h=c)
and that there are no errors in data. In particular, it follows that the hypothesis at any stage cannot
be changed by neg example.
11
Assumption: Everything except the positive
examples is negative

x1=Sunny Warm Normal Strong Warm Same +
x2=Sunny Warm High Strong Warm Same +
x3=Rainy Cold High Strong Warm Change -
x4=Sunny Warm High Strong Cool Change +
Example of Find-S
specific
general
h0=     
h1=Sunny Warm Normal Strong Warm Same
h2=Sunny Warm ? Strong Warm Same
h4=Sunny Warm ? Strong ? ?

Problems with Find-S
• Problems:
– Throws away information!
 Negative examples
– Can’t tell whether it has learned the concept
 Depending on H, there might be several h’s that fit TEs!
 Picks a maximally specific h (why?)
– Can’t tell when training data is inconsistent
 Since ignores negative TEs
• But
– It is simple
– Outcome is independent of order of examples
 Why?
• What alternative overcomes these problems?
– Keep all consistent hypotheses!
 Candidate elimination algorithm
13

Consistent Hypotheses and Version Space
• A hypothesis h is consistent with a set of training examples
D of target concept c
if h(x) = c(x) for each training example x, c(x) in D
– Note that consistency is with respect to specific D.
• Notation:
Consistent (h, D)  x, c(x)D :: h(x) = c(x)
• The version space, VSH,D , with respect to hypothesis space
H and training examples D, is the subset of hypotheses
from H consistent with D
• Notation:
VSH,D = {h | h  H  Consistent (h, D)}
14

List-Then-Eliminate Algorithm
1. VersionSpace  list of all hypotheses in H
2. For each training example x, c(x)
remove from VersionSpace any hypothesis h for which
h(x)  c(x)
3. Output the list of hypotheses in VersionSpace
4. This is essentially a brute force procedure
15

x1=Sunny Warm Normal Strong Warm Same +
x2=Sunny Warm High Strong Warm Same +
x3=Rainy Cold High Strong Warm Change -
x3=Sunny Warm High Strong Cool Change +
Example of Find-S, Revisited
specific
general
h0=     
h1=Sunny Warm Normal Strong Warm Same
h4=Sunny Warm ? Strong ? ?

Version Space for this Example
Sunny Warm ? Strong ? ?
Sunny ? ? Strong ? ? Sunny Warm ? ? ? ? ? Warm ? Strong ? ?
Sunny ? ? ? ? ? ? Warm ? ? ? ?
{ , }
G
{ }
S
17

Representing Version Spaces
• Want more compact representation of VS
– Store most/least general boundaries of space
– Generate all intermediate h’s in VS
– Idea that any h in VS must be consistent with all TE’s
 Generalize from most specific boundaries
 Specialize from most general boundaries
• The general boundary, G, of version space VSH,D is the set
of its maximally general members consistent with D
– Summarizes the negative examples; anything more general will
cover a negative TE
• The specific boundary, S, of version space VSH,D is the set
of its maximally specific members consistent with D
– Summarizes the positive examples; anything more specific will fail
to cover a positive TE
18

Theorem
• Must prove:
– 1) every h satisfying RHS is in VSH,D;
– 2) every member of VSH,D satisfies RHS.
• For 1), let g, h, s be arbitrary members of G, H, S respectively with
g>h>s
– s must be satisfied by all + TEs and so must h because it is more general;
– g cannot be satisfied by any – TEs, and so nor can h
– h is in VSH,D since satisfied by all + TEs and no – TEs
• For 2),
– Since h satisfies all + TEs and no – TEs, h  s, and g  h. 19
Every member of the version space lies between the
S,G boundary
VSH,D = {h | h  H  sS gG (g  h  s)}

Candidate Elimination Algorithm
G  maximally general hypotheses in H
S  maximally specific hypotheses in H
For each training example d, do
• If d is positive
– Remove from G every hypothesis inconsistent with d
– For each hypothesis s in S that is inconsistent with d
 Remove s from S
 Add to S all minimal generalizations h of s such that
1. h is consistent with d, and
2. some member of G is more general than h
– Remove from S every hypothesis that is more general than another
hypothesis in S
20

Candidate Elimination Algorithm (cont)
• If d is a negative example
– Remove from S every hypothesis inconsistent with d
– For each hypothesis g in G that is inconsistent with d
 Remove g from G
 Add to G all minimal specializations h of g such that
1. h is consistent with d, and
2. some member of S is more specific than h
– Remove from G every hypothesis that is less general than another
hypothesis in G
• Essentially use
– Pos TEs to generalize S
– Neg TEs to specialize G
• Independent of order of TEs
• Convergence guaranteed if:
– no errors
– there is h in H describing c.
21

Example
S0
G0 {? ? ? ? ? ?}
G1 {? ? ? ? ? ?}
{     }
S1 {Sunny Warm Normal Strong Warm Same}
Sunny Warm Normal Strong Warm Same +
Recall : If d is positive
Remove from G every hypothesis inconsistent with d
For each hypothesis s in S that is inconsistent with d
•Remove s from S
•Add to S all minimal generalizations h of s that
are specializations of a hypothesis in G
•Remove from S every hypothesis that is more
general than another hypothesis in S

G1
Sunny Warm High Strong Warm Same +
G2 {? ? ? ? ? ?}
S2 {Sunny Warm ? Strong Warm Same}
S1
{? ? ? ? ? ?}
{Sunny Warm Normal Strong Warm Same}
23
Example (contd)

S2
Rainy Cold High Strong Warm Change -
G2 {? ? ? ? ? ?}
{Sunny Warm ? Strong Warm Same}
S3
Sunny ? ? ? ? ?
{ }
, ? Warm ? ? ? ?, ? ? ? ? ? Same
G3
– For each hypothesis g in G that is inconsistent with d
 Remove g from G
 Add to G all minimal specializations h of g that generalize
some hypothesis in S
 Remove from G every hypothesis that is less general than
another hypothesis in G
Recall: If d is a negative example
– Remove from S every hypothesis inconsistent with d
24
Current G boundary is incorrect
So, need to make it more specific.
Example (contd)

 Why are there no hypotheses left relating to:
–  Cloudy ? ? ? ? ?
 The following specialization using the third value
? ? Normal ? ? ?,
is not more general than the specific boundary
 The specializations ? ? ? Weak ? ?,
? ? ? ? Cool ? are also inconsistent with S
25
Example (contd)

{Sunny ? ? ? ? ?, ? Warm ? ? ? ?, ? ? ? ? ?Same}
S3
G3
Sunny Warm High Strong Cool Change +
{Sunny Warm ? Strong ? ?}
S4
{Sunny ? ? ? ? ?, ? Warm ? ? ? ?}
G4
26
Example (contd)

 Why does this example remove a hypothesis from G?:
– ? ? ? ? ? Same
 This hypothesis
– Cannot be specialized, since would not cover new TE
– Cannot be generalized, because more general would cover negative
TE.
– Hence must drop hypothesis.
27
Sunny Warm High Strong Cool Change +
Example (contd)

Version Space of the Example
{Sunny ? ? ? ? ?, ? Warm ? ? ? ?}
G
S
28
version
space
S
G

Convergence of algorithm
• Convergence guaranteed if:
– no errors
– there is h in H describing c.
• Ambiguity removed from VS when S = G
– Containing single h
– When have seen enough TEs
• If have false negative TE, algorithm will remove every h consistent
with TE, and hence will remove correct target concept from VS
– If observe enough TEs will find that S, G boundaries converge to empty VS
29

Let us try this
30
Origin Manufacturer Color Decade Type
Japan Honda Blue 1980 Economy +
Japan Toyota Green 1970 Sports -
Japan Toyota Blue 1990 Economy +
USA Chrysler Red 1980 Economy -
Japan Honda White 1980 Economy +

And this
31
Origin Manufacturer Color Decade Type
Japan Honda Blue 1980 Economy +
Japan Toyota Green 1970 Sports -
Japan Toyota Blue 1990 Economy +
USA Chrysler Red 1980 Economy -
Japan Honda White 1980 Economy +
Japan Toyota Green 1980 Economy +
Japan Honda Red 1990 Economy -

{Sunny ? ? ? ? ?, ? Warm ? ? ? ?}
G
S
Which Next Training Example?
Assume learner can choose the next TE
• Should choose d such that
– Reduces maximally the number of hypotheses in VS
– Best TE: satisfies precisely 50% hypotheses;
 Can’t always be done
– Example:
 Sunny Warm Normal Weak Warm Same ?
 If pos, generalizes S
 If neg, specializes G
32
Order of
examples matters
for intermediate
sizes of S,G; not
for the final S, G

Classifying new cases using VS
• Use voting procedure on following examples:
 Sunny Warm Normal Strong Cool Change
 Rainy Cool Normal Weak Warm Same
 Sunny Warm Normal Weak Warm Same
 Sunny Cold Normal Strong Warm Same
{Sunny ? ? ? ? ?, ? Warm ? ? ? ?}
G
S
33

Effect of incomplete hypothesis space
• Preceding algorithms work if target function is in H
– Will generally not work if target function not in H
• Consider following examples which represent target
function
“sky = sunny or sky = cloudy”:
 Sunny Warm Normal Strong Cool Change Y
 Cloudy Warm Normal Strong Cool Change Y
 Rainy Warm Normal Strong Cool Change N
• If apply CE algorithm as before, end up with empty VS
– After first two TEs, S= ? Warm Normal Strong Cool Change
– New hypothesis is overly general
 it covers the third negative TE!
• Our H does not include the appropriate c
34
Need more
expressive
hypotheses

Incomplete hypothesis space
• If c not in H, then consider generalizing representation of H
to contain c
– For example, add disjunctions or negations to
representation of hypotheses in H
• One way to avoid problem is to allow all possible
representations of h’s
– Equivalent to allowing all possible subsets of instances as
defining the concept of EnjoySport
 Recall that there are 96 instances in EnjoySport; hence there are
296 possible hypotheses in full space H
 Can do this by using full propositional calculus with AND, OR, NOT
 Hence H defined only by conjunctions of attributes is biased
(containing only 973 h’s)
35

Unbiased Learners and Inductive Bias
• BUT if have no limits on representation of hypotheses
(i.e., full logical representation: and, or, not), can only learn
examples…no generalization possible!
– Say have 5 TEs {x1, x2, x3, x4, x5}, with x4, x5 negative TEs
• Apply CE algorithm
– S will be disjunction of positive examples (S={x1 OR x2 OR x3})
– G will be negation of disjunction of negative examples (G={not
(x4 or x5)})
– Need to use all instances to learn the concept!
• Cannot predict usefully:
– TEs have unanimous vote
– other h’s have 50/50 vote!
 For every h in H that predicts +, there is another that predicts -
36

Unbiased Learners and Inductive Bias
• Approach:
– Place constraints on representation of hypotheses
 Example of limiting connectives to conjunctions
 Allows learning of generalized hypotheses
 Introduces bias that depends on hypothesis representation
• Need formal definition of inductive bias of learning
algorithm
37

Inductive Syst and Equiv Deductive Syst
• Inductive bias made explicit in equivalent deductive system
– Logically represented system that produces same outputs
(classification) from inputs (TEs, instance x, bias B) as CE
procedure
• Inductive bias (IB) of learning algorithm L is any minimal
set of assertions B such that for any target concept c and
training examples D, we can logically infer value c(x) of
any instance x from B, D, and x
– E.g., for rote learner, B = {}, and there is no IB
• Difficult to apply in many cases, but a useful guide
38

Inductive Bias and specific learning algs
• Rote learners:
no IB
• Version space candidate elimination algorithm:
c can be represented in H
• Find-S: c can be represented in H;
all instances that are not positive are negative
39

40
Computational Complexity of VS
• The S set for conjunctive feature vectors and tree-
structured attributes is linear in the number of features and
the number of training examples.
• The G set for conjunctive feature vectors and tree-
structured attributes can be exponential in the number of
training examples.
• In more expressive languages, both S and G can grow
exponentially.
• The order in which examples are processed can
significantly affect computational complexity.

41
Exponential size of G
• n Boolean attributes
• 1 positive example: (T, T, .., T)
• n/2 negative examples:
– (F,F,T,..T)
– (T,T,F,F,T..T)
– (T,T,T,T,F,F,T..T)
– ..
– (T,..T,F,F)
• Every hypothesis in G needs to choose from n/2 2-element
sets.
– Number of hypotheses = 2n/2

Summary
• Concept learning as search through H
• General-to-specific ordering over H
• Version space candidate elimination algorithm
• S and G boundaries characterize learner’s uncertainty
• Learner can generate useful queries
• Inductive leaps possible only if learner is biased!
• Inductive learners can be modeled as equiv deductive systems
• Biggest problem is inability to handle data with errors
– Overcome with procedures for learning decision trees
42

concept-learning.ppt

More Related Content

What's hot (20)

Similar to concept-learning.ppt (20)

Recently uploaded (20)

concept-learning.ppt