# When does a UMP test fail to exist?

I have a sample $X=(X_1, ...,X_n)\sim N(\mu,\sigma^2)$ with $\sigma^2$ known. The hypotheses are $H_0: \mu=\mu_0, H_1:\mu \neq \mu_0$. I know that in such a case an UMP test does not exist and so that I should proceed using a LR test, in order to find the rejection rule.

My professor also told me that for a sample distribution that belongs to the Exponential Family in the case of simple vs bilateral hypotheses an UMP test does not exist.

Thus, my question is theoretic: why does an UMP test does not exist in such cases? Which are the conditions under which an UMP test does not exist?

EDIT: I have found an example in which, instead, although the alternative hypothesis is bilateral, the UMP test exists.

A sample $X\sim U(0,\theta)$. The hypotheses are $H_0:\theta=\theta_0, H_1:\theta\neq \theta_0$. • Simply put, a UMP test does not exist because the critical regions turn out to be different for $\theta> \theta_0$ and $\theta<\theta_0$. This means that there are only UMP tests for one-sided hypotheses, where you may still use the NP lemma. – JohnK Dec 30 '15 at 21:54
• @JohnK Ok, that's clear. But how can I apply this idea to the two examples above? How can I show that it is actually the case? – PhDing Dec 31 '15 at 8:57
• I am not familiar with a general proof. Perhaps you can work something out starting from the definition of an exponential density. My guess is that it would be non-trivial. – JohnK Dec 31 '15 at 10:57
• @Alessandro: it depends on how deep you want to dive into it. I think the paper is interesting to learn which conditions are needed and why – user83346 Jan 2 '16 at 12:10
• I thought you were interested in exponential densities. The uniform distribution does not belong to the exponential family so there is no general rule there. – JohnK Jan 3 '16 at 16:51