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Expression of IdeasVery hardSynthesis: a limitation shared by two studies

Expression of Ideas practice question

While researching a topic, a student has taken the following notes: - Study A (2019) followed 200 adults; those who drank coffee after 4 pm fell asleep 25 minutes later on average. - Study B (2022) followed 1,500 adults; it found no difference in time to fall asleep, but coffee after 4 pm reduced deep sleep by 15 percent. - Both studies were observational: participants chose for themselves whether to drink coffee.

The student wants to point out a limitation that both studies share. Which choice most effectively uses relevant information from the notes to accomplish this goal?

  1. A. Because participants in both studies chose for themselves whether to drink coffee, neither study can show that coffee caused the differences it recorded.Correct
  2. B. Study A found that coffee after 4 pm delayed sleep by 25 minutes.
  3. C. Study B, with 1,500 adults, was much larger than Study A.
  4. D. The two studies disagreed about whether coffee delays sleep.

Answer: A. Because participants in both studies chose for themselves whether to drink coffee, neither study can show that coffee caused the differences it recorded.

Self-selection limits both.

Why the other answers are wrong

B. Study A found that coffee after 4 pm delayed sleep by 25 minutes.
A finding of one study, not a limitation.
C. Study B, with 1,500 adults, was much larger than Study A.
A difference, not a shared limitation.
D. The two studies disagreed about whether coffee delays sleep.
A disagreement, not a limitation.

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