LIKELIHOOD RATIO TESTS FOR FIXED MODEL TERMS USING RESIDUAL MAXIMUM-LIKELIHOOD

Likelihood ratio tests for fixed model terms are proposed for the anal ysis of linear mixed models when using residual maximum likelihood est imation. Bartlett-type adjustments, using an approximate decomposition of the data, are developed for the test statistics. A simulation stud y is used to compare properties of the test statistics proposed, with or without adjustment, with a Ward test. A proposed test statistic con structed by dropping fixed terms from the full fixed model is shown to give a better approximation to the asymptotic chi(2)-distribution tha n the Wald test for small data sets. Bartlett adjustment is shown to i mprove the chi(2)-approximation for the proposed tests substantially.