Moments and Other Measures in Terms of Expectations

 

Moments and Other Measures in Terms of Expectations

Data Science and A.I. Lecture Series

By Bindeshwar Singh Kushwaha – PostNetwork Academy

Moments

The \( r^{th} \) order moment about any point \( A \) of a variable \( X \) is given by:

For discrete variables:

\[ \mu_r’ = \sum_{i=1}^{n} p_i (x_i – A)^r \]

For continuous variables:

\[ \mu_r’ = \int_{-\infty}^{\infty} (x – A)^r f(x) dx \]

Central Moments

The \( r^{th} \) order central moment is given by:

For discrete variables:

\[ \mu_r = \sum_{i=1}^{n} p_i (x_i – \mu)^r \]

For continuous variables:

\[ \mu_r = \int_{-\infty}^{\infty} (x – \mu)^r f(x) dx \]

Expectation form:

\[ \mu_r = E[(X – \mu)^r] \]

Variance

Variance of a random variable \( X \) is the second-order central moment:

\[ V(X) = E[X^2] – (E[X])^2 \]

Using moments about the origin:

\[ V(X) = \mu_2′ – (\mu_1′)^2 \]

Theorem: Variance Scaling Property

If \( X \) is a random variable and \( a, b \) are constants, then:

\[ V(aX + b) = a^2 V(X) \]

Proof of Theorem

By definition of variance:

\[ V(aX + b) = E[(aX + b – E[aX + b])^2] \]

Expanding expectation:

\[ = E[(aX + b – aE[X] – b)^2] \]

Simplifying:

\[ = E[a^2 (X – E[X])^2] \]

Using expectation properties:

\[ = a^2 E[(X – E[X])^2] = a^2 V(X) \]

Example: Variance Calculation

X p(X)
-2 0.15
-1 0.30
0 0
1 0.30
2 0.25

Computing \( V(X) \):

\[ V(X) = E[X^2] – (E[X])^2 \]

Using values:

\[ V(X) = 2.2 – (0.2)^2 = 2.2 – 0.04 = 2.16 \]

Variance of a Linear Transformation

Computing \( V(2X + 3) \):

Using theorem: \( V(aX + b) = a^2 V(X) \)

\[ V(2X + 3) = 4V(X) \]

Substituting \( V(X) = 2.16 \):

\[ V(2X + 3) = 4(2.16) = 8.64 \]

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