You come across a study that was designed to test whether drinking coffee improves concentration. College students were randomly assigned to either drink a cup of hot coffee or a glass of iced tea, and then they all completed a reading comprehension test. Results indicated that the coffee drinkers scored higher on reading comprehension than the iced tea drinkers, and the researchers concluded that coffee improves concentration. Which of the following is NOT a good criticism of this study?

Question

BlackTom AI · Accepted Answer

Random assignment aims to equalize pre-existing differences across groups, so claiming pre-existing differences undermines the study is not a strong criticism.

1. Relevant knowledge points and concepts
- Random assignment: in experimental design, randomly placing participants into conditions distributes both known and unknown participant characteristics roughly equally across groups, reducing selection bias.
- Internal validity: threats include confounds, demand characteristics, placebo effects, and measurement issues.
- Causation: a properly randomized experiment supports causal inference if other threats are controlled.

2. Step-by-step reasoning
- The study randomly assigned college students to coffee or iced tea. That randomization is the key control against pre-existing group differences (e.g., baseline test-taking ability).
- If randomization is properly implemented and sample size is reasonable, differences in pre-existing characteristics are expected to be small and not systematically favor one group.
- Therefore, saying “there may have been pre-existing differences between the groups” is not a strong criticism because random assignment is specifically used to prevent that problem.

3. (No numerical calculations are needed here.)

4. Why the correct answer is correct
- The statement in the correct answer describes a selection bias concern appropriate for nonrandomized studies. Because this study used random assignment, that concern is mitigated. Thus this statement is NOT a good criticism of this randomized experiment.

5. Why other plausible criticisms would be valid (briefly)
- Lack of blinding/placebo effects: participants might expect coffee to improve concentration, inflating scores—this is a good criticism.
- Differences in temperature or enjoyment (hot coffee vs iced tea) could be confounds—valid concern.
- The reading comprehension test may not accurately measure concentration—measurement validity issue.
- Sample limited to college students limits generalizability—external validity concern.
Each of these points threatens interpretation more directly than alleged pre-existing differences given the random assignment.

6. Conclusion
Because random assignment minimizes pre-existing group differences, the provided answer is correct: “There may have been pre-existing differences between the groups…” is NOT a good criticism. This matches the given answer.

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