# Fisher Information of a function of a parameter

Suppose that $X$ is a random variable for which the p.d.f. or the p.f. is $f(x|\theta)$, where the value of the parameter $\theta$ is unknown but must lie in an open interval $\Omega$. Let $I_0(\theta)$ denote the Fisher information in $X.$ Suppose now that the parameter $\theta$ is replaced by a new parameter $\mu$, where $\theta = \psi(\mu)$ and $\psi$ is a differentiable function. Let $I_1(\mu)$ denote the Fisher information in $X$ when the parameter is regarded as $\mu.$ Show that $$I_1(\mu) = [\psi'(\mu)]^2 I_0[\psi(\mu)].$$

How would I do this? Do I need to use a Taylor expansion? Regardless, I would appreciate a written proof. This isn't for class but the above statement has been mentioned in texts without any detail whatsoever.

Thanks!

• Hint: What's the definition of $I_0(\theta)$? Under standard regularity conditions, what is $\mathbb{E}\big(\frac{\partial \log f(x;\theta)}{\partial \theta}\big)$. Put this together with the chain rule of differentiation. Commented May 8, 2011 at 20:15

By definition $I_{0}(\theta)=-\mathbb{E}\left[\frac{d^{2}\log f\left(X\vert\theta\right)}{d\theta^{2}}\right].$ So $I_{1}(\mu)=-\mathbb{E}\left[\frac{d^{2}\log f\left(X\vert\mu\right)}{d\mu^{2}}\right]$.By the chain rule we have $$I_{1}\left(\mu\right) = -\mathbb{E}\left[\frac{d^{2}\log f\left(X\vert\theta\right)}{d\theta^{2}}\left(\frac{d\theta}{d\mu}\right)^{2}+\frac{d\log f\left(X\vert\theta\right)}{d\theta}\frac{d^{2}\theta}{d\mu^{2}}\right]$$ $$= -\mathbb{E}\left[\frac{d^{2}\log f\left(X\vert\theta\right)}{d\theta^{2}}\right]\left(\frac{d\theta}{d\mu}\right)^{2}+\mathbb{E}\left[\frac{d\log f\left(X\vert\theta\right)}{d\theta}\right]\frac{d^{2}\theta}{d\mu^{2}}.$$

But $\mathbb{E}\left[\frac{d\log f\left(X\vert\theta\right)}{d\theta}\right]=0.$ So we get
$$I_{1}\left(\mu\right)=I_{0}\left(\theta\right)\left(\frac{d\theta}{d\mu}\right)^{2}.$$

• Can you explain how you're using the chain rule? Which functions are involved? Commented May 9, 2011 at 1:37
• The functions involved are $\frac{d}{d\theta}\log f\left(X\vert\theta\right)$ and $\frac{d\theta}{d\mu}$ It is a product and you differentiating wrt $\mu$
– Nana
Commented May 9, 2011 at 3:10
• NB: $\frac{d^{2}\log f\left(X\vert\mu\right)}{d\mu^{2}}=\frac{d}{d\mu}\left[\frac{d}{d\mu}\log f\left(X\vert\mu\right)\right]=\frac{d}{d\mu}\left[\frac{d}{d\theta}\log f\left(X\vert\theta\right)\cdot\frac{d\theta}{d\mu}\right]$
– Nana
Commented May 9, 2011 at 3:24
• Where does 𝔼[d logf(X|θ) / dθ] = 0 come from? Commented Sep 10, 2013 at 20:43
• I think the chain rule in the comment to this answer by @Nana is incorrect. It should be $\frac{d}{d\mu}logf(X|\theta)$ in the left bracket. Commented Jul 12, 2017 at 16:04