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Problem-Solving and Data AnalysisVery hardConclusion that a volunteer trial cannot support

Problem-Solving and Data Analysis practice question

Two hundred adults who volunteered for a study were randomly assigned either to a new exercise programme or to their usual routine. After three months, the programme group had lower average blood pressure. Which of the following conclusions is NOT justified by this study?

  1. A. The programme would lower blood pressure for adults in the country as a whole.Correct
  2. B. Among these volunteers, the programme caused the lower blood pressure.
  3. C. Random assignment made the two groups alike, on average, before the study began.
  4. D. Random assignment reduced the chance that some other difference between the groups explains the result.

Answer: A. The programme would lower blood pressure for adults in the country as a whole.

Random ASSIGNMENT supports a causal conclusion for the people studied. But the participants were volunteers, not a random sample of the country, so generalising to all adults isn't justified.

Why the other answers are wrong

B. Among these volunteers, the programme caused the lower blood pressure.
This IS justified: random assignment lets the difference be attributed to the programme, for these volunteers.
C. Random assignment made the two groups alike, on average, before the study began.
This is what random assignment does.
D. Random assignment reduced the chance that some other difference between the groups explains the result.
This is the purpose of random assignment.

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