Researchers compare reading scores between students who attend a private after-school tutoring program and those who do not. Tutored students score higher on average. Tutored students also tend to come from families with higher incomes.
Why can we not conclude that tutoring caused higher scores?
- Acheck_circle
Family income is a potential confounding variable
- B
The study was double-blind
- C
The sample size is too small
- D
There was no placebo
Explanation
Income is associated with both treatment (tutoring) and outcome (scores), so its effect cannot be separated from the tutoring effect. This is confounding, fixable only with random assignment.