View source for Neyman-Pearson lemma

The '''Neyman-Pearson lemma''' is a fundamental result in statistical hypothesis testing that establishes the optimality of the likelihood ratio test. Formulated by [[Jerzy Neyman]] and [[Egon Pearson]] in 1933, the lemma proves that among all possible tests of a simple null hypothesis against a simple alternative, the test based on the likelihood ratio maximizes [[Statistical power|power]] for any given size (Type I error rate).

The lemma is not merely a technical theorem; it is the mathematical foundation of the Neyman-Pearson framework, which treats hypothesis testing as a decision problem with explicitly controlled error rates. It provided the rigorous justification for using likelihood ratios that [[Ronald Fisher]] had employed more intuitively, but with a crucial difference: Fisher used likelihood to measure evidence, while Neyman and Pearson used it to optimize decisions.

The lemma's limitation is that it applies only to simple hypotheses — fully specified distributions with no free parameters. The extension to [[Composite hypothesis|composite hypotheses]] requires additional assumptions and spawned the field of [[Uniformly Most Powerful Test|uniformly most powerful tests]].

[[Category:Mathematics]] [[Category:Statistics]]