SAT OwlQuestion bank / Evaluating statistical claims: observational studies and experiments
At a high school, students could choose whether to take an after-school study skills course. Students who took the course had a higher average score on the final exams than students who did not. The school concludes that the course caused the higher scores. Which of the following, if true, would most weaken the school's conclusion?
Some students who did not take the course scored higher than some students who took it.
Students who took the course scored higher on the final exams than they had on practice exams before the course.
Before the course began, the students who chose to take it had higher grades, on average, than the students who did not.
The course met twice a week for weeks.
Show answer and explanation
Answer: C
Because students chose whether to take the course, the two groups may have differed from the start. If the students who took the course already had higher grades, their higher exam scores could come from that difference rather than from the course.
Choice A is tempting, but the conclusion is about averages, and some overlap between individuals in two groups is expected even when one group does better on average.
Why the other choices are wrong
- A. The conclusion is about averages, and some overlap between individuals is expected.
- B. Improving after the course is consistent with the conclusion, so it doesn't weaken it.
- D. The length of the course doesn't suggest another explanation for the higher scores.
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