Russell's study compared gpa of those students who volunteered for academic study skills training and those who did not elect to take the training. he found that those who had the training also had higher gpa. with which validity threat should russell be most concerned?

Selection validity threat

Explanation: In simple words, selection validity threat refers to the threat under which the groups selected for study are not actually comparable due to some major differences in skill sets or other such factors.

In the given case, Russel study did not give any expected results of observing the students with high grade. This means the selection criteria was not accurate.

Hence the given case depicts selection threat.