  Mixed effects models are widely used to describe heterogeneity in a
population. A crucial issue when adjusting such a model to data consists in
identifying fixed and random effects. From a statistical point of view, it
remains to test the nullity of the variances of a given subset of random
effects. Some authors have proposed to use the likelihood ratio test and have
established its asymptotic distribution in some particular cases. Nevertheless,
to the best of our knowledge, no general variance components testing procedure
has been fully investigated yet. In this paper, we study the likelihood ratio
test properties to test that the variances of a general subset of the random
effects are equal to zero in both linear and nonlinear mixed effects model,
extending the existing results. We prove that the asymptotic distribution of
the test is a chi-bar-square distribution, that is to say a mixture of
chi-square distributions, and we identify the corresponding weights. We
highlight in particular that the limiting distribution depends on the presence
of correlations between the random effects but not on the linear or nonlinear
structure of the mixed effects model. We illustrate the finite sample size
properties of the test procedure through simulation studies and apply the test
procedure to two real datasets of dental growth and of coucal growth.
