Archaeologists see the value, if not the allure, of formation theory. Before inferring what happened in the past and why, we must know how the material record formed. Pottery is abundant and informative, therefore a common analytical subject. Understanding size and composition of ceramics assemblages requires formation theory, including knowledge of vessel use life. This fundamental quantity has two salient properties. The first—central tendency measured by mean or median—is widely acknowledged. Use life's second, equally important, property is the distribution of failure-age by specimen across assemblages. This article considers how and why both use-life properties affect size and composition of pottery assemblages. From a longitudinal ethnoarchaeology of household pottery in Michoacán, Mexico, it identifies vessel-size measures that correlate with use-life mean, and it demonstrates archaeologically innovative ways to characterize distributions that improve both analysis of assemblages and comparison between them.
