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Problem-Solving and Data AnalysisMediumStrengthening an association into a cause

Problem-Solving and Data Analysis practice question

A survey found that people who take vitamin D supplements report fewer colds than people who don't. Which additional finding would most strengthen the case that vitamin D CAUSES fewer colds?

  1. A. In a trial where participants were randomly assigned to take either vitamin D or a placebo, the vitamin D group had fewer colds.Correct
  2. B. Vitamin D supplements are sold in most pharmacies.
  3. C. People who take supplements also tend to exercise more than people who don't.
  4. D. Colds are more common in winter than in summer.

Answer: A. In a trial where participants were randomly assigned to take either vitamin D or a placebo, the vitamin D group had fewer colds.

Random assignment removes the differences between people who choose to take supplements and those who don't, so a difference in colds can be credited to the vitamin itself.

Why the other answers are wrong

B. Vitamin D supplements are sold in most pharmacies.
Availability says nothing about effect.
C. People who take supplements also tend to exercise more than people who don't.
This WEAKENS the case: exercise, not vitamin D, could explain the fewer colds.
D. Colds are more common in winter than in summer.
Seasonality is unrelated to whether the supplement works.

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