25
Oct 16

Properties of variance

All properties of variance in one place

Certainty is the mother of quiet and repose, and uncertainty the cause of variance and contentions. Edward Coke

Preliminaries: study properties of means with proofs.

Definition. Yes, uncertainty leads to variance, and we measure it by Var(X)=E(X-EX)^2. It is useful to use the name deviation from mean for X-EX and realize that E(X-EX)=0, so that the mean of the deviation from mean cannot serve as a measure of variation of X around EX.

Property 1. Variance of a linear combination. For any random variables X,Y and numbers a,b one has
(1) Var(aX + bY)=a^2Var(X)+2abCov(X,Y)+b^2Var(Y).
The term 2abCov(X,Y) in (1) is called an interaction term. See this post for the definition and properties of covariance.
Proof.
Var(aX + bY)=E[aX + bY -E(aX + bY)]^2

(using linearity of means)
=E(aX + bY-aEX -bEY)^2

(grouping by variable)
=E[a(X-EX)+b(Y-EY)]^2

(squaring out)
=E[a^2(X-EX)^2+2ab(X-EX)(Y-EY)+(Y-EY)^2]

(using linearity of means and definitions of variance and covariance)
=a^2Var(X) + 2abCov(X,Y) +b^2Var(Y).
Property 2. Variance of a sum. Letting in (1) a=b=1 we obtain
Var(X + Y) = Var(X) + 2Cov(X,Y)+Var(Y).

Property 3. Homogeneity of degree 2. Choose b=0 in (1) to get
Var(aX)=a^2Var(X).
Exercise. What do you think is larger: Var(X+Y) or Var(X-Y)?
Property 4. If we add a constant to a variable, its variance does not change: Var(X+c)=E[X+c-E(X+c)]^2=E(X+c-EX-c)^2=E(X-EX)^2=Var(X)
Property 5. Variance of a constant is zero: Var(c)=E(c-Ec)^2=0.

Property 6. Nonnegativity. Since the squared deviation from mean (X-EX)^2 is nonnegative, its expectation is nonnegativeE(X-EX)^2\ge 0.

Property 7. Only a constant can have variance equal to zero: If Var(X)=0, then E(X-EX)^2 =(x_1-EX)^2p_1 +...+(x_n-EX)^2p_n=0, see the definition of the expected value. Since all probabilities are positive, we conclude that x_i=EX for all i, which means that X is identically constant.

Property 8. Shortcut for variance. We have an identity E(X-EX)^2=EX^2-(EX)^2. Indeed, squaring out gives

E(X-EX)^2 =E(X^2-2XEX+(EX)^2)

(distributing expectation)

=EX^2-2E(XEX)+E(EX)^2

(expectation of a constant is constant)

=EX^2-2(EX)^2+(EX)^2=EX^2-(EX)^2.

All of the above properties apply to any random variables. The next one is an exception in the sense that it applies only to uncorrelated variables.

Property 9. If variables are uncorrelated, that is Cov(X,Y)=0, then from (1) we have Var(aX + bY)=a^2Var(X)+b^2Var(Y). In particular, letting a=b=1, we get additivityVar(X+Y)=Var(X)+Var(Y). Recall that the expected value is always additive.

GeneralizationsVar(\sum a_iX_i)=\sum a_i^2Var(X_i) and Var(\sum X_i)=\sum Var(X_i) if all X_i are uncorrelated.

Among my posts, where properties of variance are used, I counted 12 so far.

19 Responses for "Properties of variance"

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