SAT OwlQuestion bank / Evaluating statistical claims: observational studies and experiments
To estimate how often residents of a town exercise, a researcher surveyed people as they left the town's largest gym. The researcher concluded that the average town resident exercises times per week. Which of the following best explains why this conclusion is not justified?
The sample size of is too small to draw any conclusion.
The survey should have asked about exercise per month instead of per week.
People leaving a gym are likely to exercise more often than typical town residents.
The researcher should have surveyed people entering the gym instead.
Show answer and explanation
Answer: C
For a sample to represent a population, it must be selected so that every member of the population has a fair chance of being included. People leaving a gym are not representative of all residents, since they probably exercise more than average. This sampling bias means the estimate likely overstates how often residents exercise.
Choice A is tempting, but a well-chosen random sample of can be large enough; the problem is how the sample was chosen, not its size.
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