Likelihood ratio tests for fixed model terms using residual maximum likelihood

Likelihood ratio tests for fixed model terms are proposed for the analysis of linear mixed models when using residual maximum likelihood estimation. Bartlett-type adjustments, using an approximate decomposition of the data, are developed for the test statistics. A simulation study is used to compare properties of the test statistics proposed, with or without adjustment, with a Ward test. A proposed test statistic constructed by dropping fixed terms from the full fixed model is shown to give a better approximation to the asymptotic chi(2)-distribution than the Wald test for small data sets. Bartlett adjustment is shown to improve the chi(2)-approximation for the proposed tests substantially.