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Problem-Solving and Data AnalysisHardWhy attendance and weight loss may not be cause and effect

Problem-Solving and Data Analysis practice question

A gym reports that members who attend at least three times a week lose more weight than members who attend less often, and concludes that attending more often causes greater weight loss. Which of the following is the best reason the data do not establish this?

  1. A. Members who choose to attend more often may differ in other ways — such as diet or motivation — that also affect weight loss.Correct
  2. B. Weight cannot be measured accurately enough to compare the groups.
  3. C. The gym does not have enough members to draw any conclusion.
  4. D. Three times a week is an arbitrary dividing line.

Answer: A. Members who choose to attend more often may differ in other ways — such as diet or motivation — that also affect weight loss.

Attendance wasn't assigned; members chose it. Frequent attenders may also eat differently or be more motivated, and those differences could explain the weight loss.

Why the other answers are wrong

B. Weight cannot be measured accurately enough to compare the groups.
Scales measure weight well; measurement isn't the problem.
C. The gym does not have enough members to draw any conclusion.
Nothing says the gym is small, and size wouldn't fix self-selection.
D. Three times a week is an arbitrary dividing line.
Where the line is drawn doesn't create or remove the confounding.

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