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Problem-Solving and Data AnalysisMediumWhich design supports a causal conclusion

Problem-Solving and Data Analysis practice question

Four studies each compared weight change between people on two diets. In which study could a conclusion that one diet CAUSED greater weight loss be justified?

  1. A. Volunteers were randomly assigned to one of the two diets, and their weight change over six months was recorded.Correct
  2. B. Researchers recorded which diet each participant had chosen for themselves and their weight change over six months.
  3. C. People who had chosen each diet were surveyed about how much weight they had lost.
  4. D. A magazine compared the diets of its readers with those of non-readers and their reported weights.

Answer: A. Volunteers were randomly assigned to one of the two diets, and their weight change over six months was recorded.

Only random assignment makes the two groups alike in everything except the diet, so a difference in outcome can be attributed to the diet itself.

Why the other answers are wrong

B. Researchers recorded which diet each participant had chosen for themselves and their weight change over six months.
People who choose a diet differ in other ways (motivation, habits), so the diet can't be isolated as the cause.
C. People who had chosen each diet were surveyed about how much weight they had lost.
Self-chosen diets and self-reported losses: no assignment, and unreliable measurement too.
D. A magazine compared the diets of its readers with those of non-readers and their reported weights.
Readers and non-readers differ in many ways; nothing here isolates the diet.

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