The development of critical thinking skills is a fundamental educational objective and is regarded as an essential component of the general education of college students. To further this objective, this study investigates instructional strategies for improving the thinking skills of college students engaged in collaborative learning endeavors. Specifically, the study summarizes the results of a two-phase research project to advance knowledge about using two interventions--revision in writing and a critical thinking rubric--for improving students' group critical thinking skills as exhibited in a complex group policy paper on a controversial social issue. The study contributes methodologically to the holistic assessment of group critical thinking as well as to the use of formative assessment data for revising a curriculum, improving instruction, and enhancing student learning. (Contains 2 figures.)