Probability models for duration have been applied to a wide range of individual-level and organizational phenomena. Seemingly similar models may, however, produce different results. Building on previous work by Morrison & Schmittlein (1980) and Schmittlein and Morrison (1983), we propose a hierarchy of models and show how an analysis of the results across models helps explain why different studies came to different conclusions. This analysis also leads to substantive insights that even the most complete model alone does not provide.