Software requirements quality affects software product quality. For high-quality software products, software requirements must be complete. When faced with incomplete requirements, software engineers attempt to fill the requirements' gaps differently, either by getting feedback from the user or by making assumptions. Assumptions may be explicit or implicit. Explicit assumptions are preferable to implicit assumptions as explicit assumptions can be validated. We conduct an empirical study to determine whether the number of explicit assumptions made by software engineers is related to a project's characteristics. Using data from two CMMI Level 3 companies and 16 projects, we investigated the responses of 92 software engineers to the same incomplete software requirement. Our findings show possible relationships between projects' characteristics and the number of explicit assumptions. ©2009 IEEE.