We have studied the types of student teams that are most effective for collaborative learning in a large freshman university physics course. We compared teams in which the students were all of roughly equal ability to teams with a mix of student abilities, we compared teams with three members to teams with four members, and we examined teams with only one female student and the rest of the students male. We measured team effectiveness by the gains on the Force Concept Inventory and by performance on the final examination. None of the factors that we examined had significant impact on student learning. We also investigated student satisfaction as measured by responses to an anonymous evaluation at the end of the term, and found small but statistically significant differences depending on how the nine teams in the group were constructed.