This advice is situated in a self-tasking dossier aimed at formulating recommendations to optimize the methodology of the control program of the FASFC in order to enable the study of trends. In this regard a distinction has to be made between trend watching and trend analysis. It is examined to what extent the results of the control program can be used for trend analysis. On one hand, a model is developed to calculate the sampling sizes needed in order to determine statistically, at a particular assumed initial prevalence, the existing of a trend (trend analysis). On the other hand, a model is developed to calculate the sample sizes in order to confirm, with a particular confidence, an observed trend in a particular prevalence during subsequent years. These models are applied to several case studies. Finally, the Scientific Committee recommends to include trend watching or trend analysis for appropriate contaminant/matrix combinations in the long term evaluation of the results of the control program. ; BE; nl; secretariaat.scicom@favv-afsca.be