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Variance of a normal with random mean and scale

Question

Let MM and SS be independent Exp(1)\text{Exp}(1) random variables. Conditional on M=μM=\mu and S=σS=\sigma, the pair (X,Y)(X,Y) is bivariate normal with both means equal to μ\mu, both variances equal to σ2\sigma^2, and correlation ρ\rho, where ρ∈[−1,1]\rho\in[-1,1] is a fixed constant. Find Var(Y)\text{Var}(Y).

Short answer

33. By the law of total variance, Var(Y)=E[Var(Y∣M,S)]+Var(E[Y∣M,S])=E[S2]+Var(M)\text{Var}(Y)=\mathbb{E}[\text{Var}(Y\mid M,S)]+\text{Var}(\mathbb{E}[Y\mid M,S])=\mathbb{E}[S^2]+\text{Var}(M). For Exp(1)\text{Exp}(1) these are 22 and 11; ρ\rho plays no role.

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