What does "in studies of" mean? Is it a common expression?
"In studies of" is not necessarily a general expression, it has been used by the speaker here to describe the content of the questions of other studies. When the speaker says "in studies of similar questions," he means "in studies that consisted of similar questions." You can also rearrange the sentence to be, "up to 80 percent of participants chose the equivalent of the second statement in studies with similar questions." "In" and "studies" is an expression commonly found in sentences explaining study results. Ex: In the study, 80% of participants chose the second option. Ex: Similar results were found in multiple studies on the same topic.