Understanding experimental design is essential to all fields of science. This fundamental skill is taught beginning in elementary school and continuing through to the college classroom. Although students quickly grasp the general idea of creating an experiment to test a hypothesis, they are frequently unable to put these ideas into practice. Our aim was to develop a twenty minute in-class group activity suitable for the large lecture hall which would promote student understanding of experimental design. We designed two alternative activities to test the hypothesis that analysis of well-designed and confounded experiments would be more effective at teaching experimental design than developing a hypothesis and designing an experiment to test that hypothesis. Students in an introductory biology course were randomly assigned one of the two activities which they completed in groups of two to four during the same class period. To assess the effectiveness of these alternative interventions we measured student performance using the open-ended experimental design ability test (EDAT) administered before and after the in-class activity. The EDAT and scoring rubric were expanded in order to capture studentsâ€™ ability to justify their proposed experimental design. Comparison of change in pre/post EDAT scores using a general linear model to control for student academic ability and ethnicity revealed no significant differences between the two interventions. However, both activities increased student understanding of experimental design relative to a control group which received a traditional lecture in place of the activities (effect size = 0.25; p = 0.047). Moreover, the median performance of students who received either activity exceeds the median performance of graduate students on the same test (p = .01). Thus, creating a well-controlled experiment and justifying the design elements remains a challenge for entry-level Ph.D. students in biology.