Dr. Wohn is interested in the effects of a trust-building game (i. e. a ropes course) on the social adjustment of incoming first-year students. She goes back to records of Freshman Orientation weekend and finds 50 students who chose to do the trust-building exercise and 50 students who chose to do another activity instead (i. e. a non-trust building game). She administers a survey and finds that, on average, the students who did the trust-building game reported better social adjustment than those who did not. Dr. Wohn concludes that the trust-building exercise caused better social adjustment for those students. What is wrong with her conclusion