They compared teams in which the students were all of roughly equal ability to teams with a mix of student abilities, they compared teams with three members to teams with four members, and they examined teams with only one female student and the rest of the students male. They 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. They 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.
The full paper is available at The American Physical Society web site, Physical Review Physics Education Research: https://journals.aps.org/prper/abstract/10.1103/PhysRevPhysEducRes.12.010138#fulltext