In this note I amend and clarify the framework for cost analysis developed and applied by Morrison and Schwartz (M&S) in a series of papers. I propose a consistent decomposition of a cost-based measure of short-run returns to scale and develop a version of M&S's dynamic decomposition that makes transparent its nature and usefulness as a cost-accounting device.This research has been financed in part by the Spanish Ministry of Education and Culture through DGICYT grants no. PB95-0130 and SEC99-1189.