Stone tools were knapped, not built. This truism means that tools were reduced from larger pieces in the production process. But many tools were further reduced in use, to repair damage or as edges dulled. Reduction reduced size (trivially), but also changed the proportions among tools' elements or dimensions. Such allometric variation (change in proportion as a function of change in size) is useful to estimate the degree of reduction that tools experienced. Reduction itself is a measure of curation, a theoretical concept of great interest in lithic studies and Paleolithic archaeology. To determine the reduction that archaeological tools experienced, we must compare their size and proportions at first use to the same properties at discard that we directly measure. By now many size estimates can be made from discarded tools. Some are experimentally tested but few are validated using direct ethnoarchaeological controls. We validate two allometric reduction measures-ratios of plan area to thickness and of an estimate of original to discarded volume-against direct measures of use and reduction in ethnographic Gamo hidescrapers from Ethiopia.