Problem-Solving and Data AnalysisVery hardTwo flaws in a causal claim
Problem-Solving and Data Analysis practice question
A magazine posted a survey online for its readers. Among readers who responded, those who said they drank green tea every day also reported having fewer colds. The magazine concluded that drinking green tea prevents colds in the general population. Which of the following best describes the flaws in this conclusion?
- A. The sample was self-selected, and the survey design cannot separate the effect of green tea from other differences between the two groups of readers.Correct
- B. The sample was too small for any conclusion to be drawn.
- C. Colds cannot be counted reliably, so no survey about them can be trusted.
- D. The magazine should have reported a larger margin of error.
Answer: A. The sample was self-selected, and the survey design cannot separate the effect of green tea from other differences between the two groups of readers.
Two problems: respondents chose themselves (so they may not resemble the general population), and nobody was assigned to drink tea, so tea-drinkers may differ in other ways — diet, sleep, exercise — that explain the fewer colds.
Why the other answers are wrong
- B. The sample was too small for any conclusion to be drawn.
- The survey's size isn't given, and size isn't the issue; who was sampled and how they were compared are.
- C. Colds cannot be counted reliably, so no survey about them can be trusted.
- Self-reported colds are imperfect, but that isn't the flaw that breaks this particular conclusion.
- D. The magazine should have reported a larger margin of error.
- A margin of error describes sampling variability in a random sample; it can't fix a self-selected sample or a causal leap.
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