To investigate the variation in active learning used in college science courses, the authors analyzed 57 comparison studies published in three prominent science education journals. Focusing on three sources of variation--(a) the active learning activities, (b) other pedagogical features, and (c) course structure/design--they found that most courses contained a significant lecture component despite the "active learning and lecture" dichotomy and that courses varied widely in terms of the intensity of the active learning strategies, the time students engaged with the content, and where other activities were added. A taxonomy for active learning is also presented to help guide the design of an active learning course.