At one ACT test location students taking the test for a second time volunteered to inhale supplemental oxygen for 5 minutes before the test. In fact, some received oxygen, but others (randomly assigned) were given just normal air. Test results showed that 42 of 66 students who breathed oxygen improved their ACT scores, compared to only 35 of 63 students who did not get the oxygen.
A) Which procedure should we use to see if there is evidence that breathing extra oxygen can help test-takers think more clearly?