The typical procedure for evaluating the performance of different objective quality metrics and indices involves comparisons between subjective quality ratings and the quality indices obtained using the objective metrics in question on the known video sequences. Several correlation indicators can be employed to assess how well the subjective ratings can be predicted from the objective values. In this paper, we give an overview of the potential sources for uncertainties and inaccuracies in such studies, related both to the method of comparison, possible inaccuracies in the subjective data, as well as processing of subjective data. We also suggest some general guidelines for researchers to make comparison studies of objective video quality metrics more reliable and useful for the practitioners in the field.