Sound empirical research suggests that we should analyze software metrics from a theoretical and practical perspective. This paper describes the result of an investigation into the respective merits of two cohesion-based metrics for program slicing. The Tightness and Overlap metrics were those originally proposed by Weiser for the procedural paradigm. We compare and contrast these two metrics with a third metric for the OO paradigm first proposed by Counsell et al. based on Hamming Distance and based on a matrix-based notation. We theoretically validated the three metrics using the properties of Kitchenham and then empirically validated the same three metrics; some revealing properties of the metrics were found as a result. In particular, that the OO-based metric was the most stable of the three; module length was not a confounding factor for the Hamming Distance-based metric; it was however for the two slice-based metrics supporting previous work by Meyers and Binkley. The number of module slices however, was found to be an even stronger influence on the values of the two slice-based metrics, whose near perfect correlation with each other suggests that they may be measuring the same software attribute. We calculated and then compared the three metrics using first, a set of manufactured, pre-determined modules as a preliminary analysis and second, approximately nine thousand functions from the modules of multiple versions of the Barcode system, used previously by Meyers and Binkley in their empirical study. The over-arching message of the research is that a combination of theoretical and empirical analysis can help significantly in comparing the viability and indeed choice of a metric or set of metrics. More specifically, although cohesion is a subjective measure, there are certain properties of a metric that are less desirable than others and it is these 'relative' features that distinguish metrics, make their comparison possible and their value more evident.