In statistics, the likelihood-ratio test assesses the goodness of fit of two competing statistical models based on the ratio of their likelihoods, specifically one found by maximization over the entire parameter space and another found after imposing some constraint. If the constraint (i.e., the null hypothesis) is supported by the observed data, the two likelihoods should not differ by more than sampling error. Thus the likelihood-ratio test tests whether this ratio is significantly different from one, or equivalently whether its natural logarithm is significantly different from zero. The likelihood-ratio test, also known as Wilks test, is the oldest of the three classical approaches to hypothesis testing, together with the Lagrange multiplier test and the Wald test. In fact, the latter two can be conceptualized as approximations to the likelihood-ratio test, and are asymptotically equivalent. In the case of comparing two models each of which has no unknown parameters, use of the likelihood-ratio test can be justified by the Neyman–Pearson lemma. The lemma demonstrates that the test has the highest power among all competitors. Definition General Suppose that we have a statistical model with parameter space . A null hypothesis is often stated by saying that the parameter is in a specified subset of . The alternative hypothesis is thus that is in the complement of , i.e. in , which is denoted by . The likelihood ratio test statistic for the null hypothesis is given by: where the quantity inside the brackets is called the likelihood ratio. Here, the notation refers to the supremum. As all likelihoods are positive, and as the constrained maximum cannot exceed the unconstrained maximum, the likelihood ratio is bounded between zero and one. Often the likelihood-ratio test statistic is expressed as a difference between the log-likelihoods where is the logarithm of the maximized likelihood function , and is the maximal value in the special case that the null hypothesis is true (but not necessarily a value that maximizes for the sampled data) and denote the respective arguments of the maxima and the allowed ranges they're embedded in. Multiplying by −2 ensures mathematically that (by Wilks' theorem) converges asymptotically to being ²-distributed if the null hypothesis happens to be true. The finite sample distributions of likelihood-ratio tests are generally unknown. The likelihood-ratio test requires that the models be nested – i.e. the more complex model can be transformed into the simpler model by imposing constraints on