In deciding to accept or reject the null hypothesis, an experimenter might be making a mistake. Usually, hypothesis tests are evaluated and compared through their probabilities of making mistakes. In this lesson, the different results of a hypothesis test are discussed and the error probabilities are defined. In particular, the power function is defined. Following that, the level or size of the test is defined. Expressions for the level of the likelihood ratio tests of the Poisson rate and the size of the likelihood ratio test of a gamma scale are illustrated.