# Show that if $X\sim\text{Pois}(\theta)$, then $I(\theta;X) = 1/\theta$.

The Fisher information matrix $I(\theta;X)$ about $\theta$ based on $X$ is defined as the matrix with elements $$I_{i,j}(\theta;X) = \operatorname{Cov}_\theta\bigg(\frac{\partial}{\partial \theta_i}\log f_X(X\mid\theta), \frac{\partial}{\partial\theta_i}\log f_X(X\mid \theta) \bigg).$$

Exercise: Let $X\sim \text{Pois}(\theta)$. Show that $I(\theta;X) = 1/\theta$.

What I've tried: If I'm not mistaken then $\dfrac{\partial}{\partial \theta_i} \log f_X(X \mid \theta) = \dfrac{x_i}{\theta} + \log e$. The Fisher information matrix (a $1\times 1$ matrix in this example) would be given by $\operatorname{Cov}_\theta\left(\dfrac{x_i} \theta + \log e, \dfrac{x_i} \theta + \log e\right)$. We know that $\operatorname{Cov}_\theta\left(\dfrac{x_i} \theta + \log e, \dfrac{x_i}{\theta} + \log e\right) = \operatorname{Var}_\theta\left(\dfrac{x_i}\theta + \log e\right) = \operatorname{Var}_\theta \left(\dfrac{x_i}{\theta}\right) = x_i^2 \operatorname{Var}_\theta \left(\dfrac 1 \theta \right).$ Obviously I'm doing something wrong as the $x_i^2$ before the variance is a problem. Besides that, I'm not sure if I can calculate $\operatorname{Var}_\theta\left(\dfrac 1 \theta \right)$.

Question: How do I show that $I(\theta;X) = 1/\theta$?

Thanks in advance!

• $\operatorname{Var}_\theta \left(\dfrac{x_i}{\theta}\right) = x_i^2 \operatorname{Var}_\theta \left(\dfrac 1 \theta \right)$ doesn't make sense. You need $\operatorname{Var}_\theta \left(\dfrac{x_i}{\theta}\right) = \dfrac 1 {\theta^2} \operatorname{Var}_\theta (x_i). \qquad$ – Michael Hardy Dec 11 '17 at 16:52
• @MichaelHardy Why is that so? It's the variance w.r.t. $\theta$ right? Don't we use $\text{Var}_X$ so show that the variance is to be taken w.r.t. $X$? – titusAdam Dec 11 '17 at 16:59
• $\theta$ is a parameter and so it is just a constant. The notation $E_\theta$ or $\operatorname{Var}_\theta$ just lists the relevant parameter(s) in the subscript. The random quantities are still $X$ and all things derived from it. – Kim Jong Un Dec 11 '17 at 17:02
• @KimJongUn What would relevant mean in this sense? I'd say it's rather confusing. When taking the expectation $\operatorname{E}_X$ we mean that we take the expectation wrt $X$ right? – titusAdam Dec 11 '17 at 17:05
• @titusAdam : No. I don't take expected values or variances "with respect to" anything. Rather $\operatorname{Var}_\theta$ means the variance given that the value of the parameter is $\theta. \qquad$ – Michael Hardy Dec 11 '17 at 17:05

## 1 Answer

The mistake is in the last step: $$\operatorname{Var}(x_i/\theta) {{}\color{red}={}} x_i^2\operatorname{Var}(1/\theta).$$ Instead, it should be $$\operatorname{Var}(x_i/\theta)=\operatorname{Var}(x_i)/\theta^2=\theta/\theta^2=1/\theta.$$

• When you used \text{Var} instead of \operatorname{Var} then you don't get proper spacing in things like $3\operatorname{Var}(X)$ or $3\operatorname{Var} X.$ Instead you see $3\text{Var}(X)$ or $3\text{Var} X.$ I edited accordingly. $\qquad$ – Michael Hardy Dec 11 '17 at 16:55
• @MichaelHardy Thanks Mike. Glad to learn something new. – Kim Jong Un Dec 11 '17 at 16:56