Inaccurate or imprecise assessment of individual contributions by the teacher in a collaborative learning environment leaves the free-riding students and non-collaborating student groups undetected because of lack of accountability. This discourages student collaboration and student learning. To accurately evaluate students' contributions and to provide timely interventions, a teacher could use a summary of the tracked collaborative activities of the participating students. We have developed and deployed an individual contribution assessment method for the students collaborating in Clas-sroomWiki - a Web-based collaborative writing environment. Our contribution assessment method combines the (1) tracked useful textual contributions, (2) idea- and concept-related contributions, and (3) other contributions like peer-rating to generate a summary that assesses the members' individual contributions toward their groups. To test the impact of our contribution assessment method, we have used it to assess the contributions of the students participating in a collaborative writing assignment in two separate university-level courses using the ClassroomWiki environment. The results of our two-semester long deployment suggest that, using this summary, a teacher can (1) better evaluate the individual contribution of a student and (2) provide timely and proactive interventions to the non-contributing members and groups to improve collaboration and performance in the ClassroomWiki collaborative writing environment.