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Let X1, X2... be iid Bernoulli random variables with parameter 1/4, let Y1, Y2... be another sequence of iid Bernoulli random variables with parameter 3/4 and let N be a geometric random variable with parameter 1/2. Assume the Xi's, Yj's and N are all independent. Compute Covariance(Σ Xi,Σ Yi), i is from 1 to N.

Since X and Y are independent random variables, I concluded that Covariance of the two terms is 0. However, I strongly feel that the mixed distribution has a part to play here. Since both the distributions have n as a common parameter, can they still be considered independent?

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  • $\begingroup$ To summarise, you are asking us to find the covariance of two independent random variables? $\endgroup$ – Angina Seng Jun 24 at 2:33
  • $\begingroup$ X and Y both have n as a common parameter, can they still be considered independent? @AnginaSeng. Okay, I think I should post the exact question here. Let me edit it $\endgroup$ – Nisha Jun 24 at 2:35
  • $\begingroup$ What are $X$ and $Y$? Sums of $X_i$ and $Y_i$? In any case sharing n is irrelevant, so co-variances are $0$. $\endgroup$ – herb steinberg Jun 24 at 3:02
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This is one of those situations where it is clearer to write the limits of summation: let $$S = \sum_{i=1}^N X_i, \quad T = \sum_{i=1}^N Y_i.$$ Then they certainly have positive covariance because they both depend on $N$ in the same way. Had the upper limits been fixed, say at $5$ for $X_i$ and $11$ for $Y_i$, then the covariance is zero. But as the question is framed, knowledge of $S$, for instance, carries some information about $N$ when it is unknown, hence informs the value of $T$; e.g., observing $S = 5$ means $N \ge 5$.

To compute the covariance, it is useful to employ the law of total expectation. We first compute $$\operatorname{E}[S] = \operatorname{E}[\operatorname{E}[S \mid N]] = \operatorname{E}[N p_x] = \frac{p_x}{\theta},$$ where I have taken $p_x$ to be the Bernoulli parameter for $X_i$, and I am using the parametrization $$\Pr[N = n] = (1-\theta)^{n-1} \theta, \quad n \in \{1, 2, \ldots\},$$ so $\theta$ is the geometric distribution parameter. Similarly, $$\operatorname{E}[T] = \frac{p_y}{\theta}.$$ We now turn our attention to the computation of $$\operatorname{E}[ST] = \operatorname{E}[\operatorname{E}[ST \mid N]].$$ Note that since $N$ is given, the inner expectation is simply $$\operatorname{E}[ST \mid N] = \operatorname{E}[S \mid N]\operatorname{E}[T \mid N] = Np_x Np_y = N^2 p_x p_y.$$ Then the outer expectation with respect to $N$ is $$\operatorname{E}[ST] = p_x p_y \operatorname{E}[N^2] = \frac{2-\theta}{\theta^2} p_x p_y.$$ Consequently, $$\operatorname{Cov}[S,T] = \operatorname{E}[ST] - \operatorname{E}[S]\operatorname{E}[T] = \frac{2-\theta}{\theta^2} p_x p_y - \frac{p_x p_y}{\theta^2} = \frac{1-\theta}{\theta^2} p_x p_y.$$ It is important to understand here that this is the unconditional covariance of $S$ and $T$, rather than $$\operatorname{Cov}[S \mid N, T \mid N] = 0.$$

Had we used the alternative parametrization for $N$ $$\Pr[N = n] = (1-\theta)^n \theta, \quad n \in \{0, 1, 2, \ldots\},$$ then we would need to adjust accordingly to obtain $$\operatorname{Cov}[S,T] = \frac{2(1-\theta)^2}{\theta^2} p_x p_y.$$


Here is Mathematica code to simulate the covariance in the case $p_x = 1/4$, $p_y = 3/4$, $\theta = 1/2$ as stated in the question, with the number of simulations equal to $10^6$:

d = ParallelTable[{RandomVariate[BinomialDistribution[#, 1/4]], 
    RandomVariate[BinomialDistribution[#, 3/4]]} 
    &[RandomVariate[GeometricDistribution[1/2]] + 1], {10^6}];

Mean[Times @@ # & /@ d] - Times @@ (Mean /@ Transpose[d]) // N

This gave me a result equal to $0.375653$, which is close to the theoretical result $3/8$. The same can be done with the alternate parametrization by removing the +1. It just so happens that for the given choice of parameters, the covariance is again $3/8$, but when $\theta \ne 1/2$, the two covariances will not be the same.

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  • $\begingroup$ Yes, thankyou for making the point clear as to why the covariance is not 0. $\endgroup$ – Nisha Jun 24 at 6:04
  • $\begingroup$ Could you also explain this step: Note that since N is given, the inner expectation is simply E[ST∣N]=E[S∣N]E[T∣N]=NpxNpy=N2pxpy. How do you conclude that the two random variables are independent? $\endgroup$ – Nisha Jun 24 at 6:04
  • $\begingroup$ @Nisha the unconditional random variables $S$ and $T$ are not independent as they both depend on $N$. But once $N$ is fixed, the conditional random variables $S \mid N$ and $T \mid N$ are independent because the $X_i$s and $Y_i$s are independent. The link between the outcomes of $S$ and $T$ is broken once you fix $N$ in advance. This is why $\operatorname{E}[ST \mid N] = \operatorname{E}[S \mid N]\operatorname{E}[T \mid N]$, but $\operatorname{E}[ST] \ne \operatorname{E}[S]\operatorname{E}[T]$. $\endgroup$ – heropup Jun 24 at 6:11
  • $\begingroup$ All points clarified. Thankyou! $\endgroup$ – Nisha Jun 24 at 6:23

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