Both substantive (i.e. factual knowledge, concepts, laws and theories) and procedural knowledge (understanding and applying concepts such as reliability and validity, measurement and calibration, data collection, measurement error, the ability to interpret evidence and the like) are involved in carrying out an open-ended science investigation. There is some debate as to whether procedural understanding is of little importance compared to substantive understanding or whether--and this is the view we take--procedural ideas can and should be taught explicitly. We present here findings from a study of undergraduate students who took a module which specifically taught procedural ideas. We employ an innovative method, Charles Ragin's Qualitative Comparative Analysis (QCA), which involves the analysis of necessary and sufficient conditions and conjunctions of causes. Findings from a comparison of the students' performance before and after the teaching and from QCA imply that procedural understanding was indeed a necessary condition for carrying out an open-ended investigation. It was also sufficient when combined with either substantive understanding, prior attainment or both. (Contains 9 tables, 3 figures, and 9 notes.)