Higher education goals include helping students develop evidence based reasoning skills; therefore, scientific thinking skills such as those required to understand the design of a basic experiment are important. The Experimental Design Ability Test (EDAT) measures students' understanding of the criteria for good experimental design through their open-ended response to a prompt grounded in an everyday life science problem. Using a straightforward scoring rubric to analyze student responses, the EDAT provides for consistent and rapid evaluation. Minimal student and classroom time is required to administer the EDAT and it can be used in a pre-/posttest format to measure gains. Significantly, the EDAT is content and terminology independent, and requires minimal quantitative skills. Our findings indicate that the EDAT is sensitive to improvements in experimental design ability, as only students in our sample who participated in a redesigned introductory biology course that included explicit instruction and experiences using the scientific method, made significant gains in their experimental design ability. (Contains 4 tables and 5 figures.)