A covariate describes a property of the site, for example habitat type with the categories woodland and farmland. Incorporating a covariate in the model implies that missing counts are estimated (imputed) for each covariate category separately. Thus, a missing count in a woodland site is estimated from the changes in the other woodland sites only and a missing count in a farmland site is derived from the changes in other farmland sites only. When different trends per habitat are to be expected, this will lead to a considerable improvement of the imputations and of the fit of the model. Both the linear trend model and the time effect model can be extended by covariates.
