He can then isolate the effect of having more friends of the opposite gender on school performance.

他排除了学生交更多异性朋友对其学业成绩的影响。

Friendship groups are not random, which makes it tricky to isolate the effect of fraternising with the opposite sex on school performance.

朋友圈的非随机性使得单独考虑异性友谊对学业成绩的影响十分困难。