Faculty and students are equally concerned about and frustrated with students who don't do their fair share of work in groups. For faculty, it's a concern that prevents them from using group work. With five, 10, or maybe even 20 different groups working on a project, faculty can't possibly know what's going on in every group. And students often don't report that somebody didn't do the work until the project has been submitted. At that point it's too late to do anything that might address the issue, which leaves the teacher to figure out if it's fair to penalize the free rider based on secondhand information.