Coherence equals cohesion-or does it?

Traditionally, cohesion of a software component is evaluated as the similarity of its constituent parts. It may be more appropriate to measure functional coherence, or coherence of usage of a component, by analyzing external usage patterns of component's clients. We define an appropriate measure, starting from a generic system model and its associated mechanism for calculating object sizes. We show that the new measure satisfies some, but not all, of the properties that a cohesion measure is expected to satisfy. We also provide some intuitive examples to illustrate the concept and its possible uses.