# #StackBounty: #prior #uninformative-prior #invariance #objective-bayes Significance of parameterisation invariance of Jeffreys prior

### Bounty: 50

I often hear it said that the Jeffreys prior is well-motivated because it is invariant under reparametrization. The proof of this is quite straight-forward (I know the proof on e.g., wiki). I’m a bit confused about what the proof really means, though, because the kind of invariance proven is a bit strange to me. It is indeed proven that if
$$p(x) propto sqrt{I(x)}$$
then
$$p(y) propto sqrt{I(y)}$$
where $$I$$ is the Fisher information and $$y$$ was found through a bijective transformation of $$x$$. Note well that $$I(x)$$ is an abuse of notation, as it contains derivatives wrt the variable $$x$$.

I don’t see this as particularly compelling, since I make a similar argument that any choice of prior is parametrization invariant. E.g., by writing an arbitrary prior as
$$p(theta) = frac{dF(theta)}{dtheta}$$
where $$F$$ is the cumulative distribution function, we then find
$$p(phi) = frac{dF(phi)}{dphi}$$
To put it another way, I can specify a prior by specifying a cdf rather than a pdf, and the cdf transforms trivially under reparameterizations. This kind of invariance is of basically no interest to me.

So, why do people make a fuss about the Jeffreys prior being invariant under reparameterization? I think I would rather say that the kind of invariance that the Jeffreys prior has is necessary for any objective formal rule for selecting a prior, but not in itself a motivation for using a Jeffreys prior. And I think it would be better to say that the Jeffreys rule for making a prior was parameterisation invariant, than say the Jeffreys prior was parameterisation invariant. Is that fair?

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