Probability mass functions and probability density functions
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This document discusses probability mass functions (pmf) and probability density functions (pdf) for discrete and continuous random variables. A pmf fX(x) gives the probability of a discrete random variable X taking on the value x. A pdf fX(x) defines the probability that a continuous random variable X falls within an interval via its cumulative distribution function FX(x). The pdf must be non-negative and have an area/sum of 1 under the curve/over all x values.
![Probability Mass Functions and Probability Density Functions
The probability mass function or pmf, fX (x) of a discrete random vari-
able X is given by fX (x) =P(X = x) for all x.
The probability density function or pdf, fX (x) of a continuous random
x
variable X is the function that satisfies FX (x) = −∞ fX (t)∂t for all x
A widely accepted convention which we will adopt, is to use an uppercase
letter for the cdf and a lowercase letter for the pmf or pdf.
We must be a little more careful in our definition of a pdf in the continuous
case. If we try to naively calculate P(X = x) for a continuous random variable
we get the following:
Since {X = x} ⊂ {x − < X ≤ x} for any > 0, we have from Theorem
2(3), P{X = x} ≤P{x − < X ≤ x} = FX (x) − FX (x − ) for any > 0.
Therefore, 0 ≤ P{X = x} ≤ lim [FX (x) − FX (x − )] = 0 by the continuity
→0
of FX .
A note on notation: The expression “X has a distribution given by FX (x)”
is abbreviated symbolically by “X ∼ FX (x),” where we read they symbol “∼”
as is distributed as” or “follows”.
Theorem 5: A function fX (x) is a pdf or pmf of a random variable X if
and only if:
(1) fX (x) ≥ 0 for all x
∞
(2) −∞ fX (x)∂x = 1 (pdf) and X f (x) = 1 (pmf)
x
1](https://image.slidesharecdn.com/probabilitymassfunctionsandprobabilitydensityfunctions-121202055220-phpapp01/85/Probability-mass-functions-and-probability-density-functions-1-320.jpg)
Probability mass functions and probability density functions
- 1. Probability Mass Functions and Probability Density Functions The probability mass function or pmf, fX (x) of a discrete random vari- able X is given by fX (x) =P(X = x) for all x. The probability density function or pdf, fX (x) of a continuous random x variable X is the function that satisfies FX (x) = −∞ fX (t)∂t for all x A widely accepted convention which we will adopt, is to use an uppercase letter for the cdf and a lowercase letter for the pmf or pdf. We must be a little more careful in our definition of a pdf in the continuous case. If we try to naively calculate P(X = x) for a continuous random variable we get the following: Since {X = x} ⊂ {x − < X ≤ x} for any > 0, we have from Theorem 2(3), P{X = x} ≤P{x − < X ≤ x} = FX (x) − FX (x − ) for any > 0. Therefore, 0 ≤ P{X = x} ≤ lim [FX (x) − FX (x − )] = 0 by the continuity →0 of FX . A note on notation: The expression “X has a distribution given by FX (x)” is abbreviated symbolically by “X ∼ FX (x),” where we read they symbol “∼” as is distributed as” or “follows”. Theorem 5: A function fX (x) is a pdf or pmf of a random variable X if and only if: (1) fX (x) ≥ 0 for all x ∞ (2) −∞ fX (x)∂x = 1 (pdf) and X f (x) = 1 (pmf) x 1