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Problem-Solving and Data AnalysisVery hardStrongest evidence for a causal link

Problem-Solving and Data Analysis practice question

Surveys consistently find that students who sleep more score higher on exams. Which of the following would provide the STRONGEST evidence that more sleep actually causes higher scores?

  1. A. An experiment in which students randomly assigned to a longer-sleep schedule scored higher than students assigned to their usual schedule.Correct
  2. B. A much larger survey finding the same association.
  3. C. The same association appearing in surveys from several different countries.
  4. D. A biological theory explaining how sleep could improve memory.

Answer: A. An experiment in which students randomly assigned to a longer-sleep schedule scored higher than students assigned to their usual schedule.

Only random assignment rules out the other differences between students who happen to sleep more and those who don't. A bigger or more widespread association is still just an association.

Why the other answers are wrong

B. A much larger survey finding the same association.
A larger survey measures the same association more precisely, but can't separate sleep from everything that goes with it.
C. The same association appearing in surveys from several different countries.
The same association in many places is still an association.
D. A biological theory explaining how sleep could improve memory.
A theory suggests a mechanism but doesn't test it.

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