Question 57·Hard·Evaluate Statistical Claims: Observational Studies and Experiments
A clinical trial recruited 200 volunteers who all had chronic insomnia. The volunteers were randomly assigned to receive either a new herbal supplement or a placebo for 8 weeks. At the end of the study, the average nightly sleep duration increased significantly more in the supplement group than in the placebo group.
Which conclusion is best supported by the results of this study?
For SAT questions about studies and conclusions, first identify the design: random assignment (experiment) allows causal conclusions; random sampling allows you to generalize to a wider population. Then check the answer choices for two things: (1) whether they use causal words like “causes” appropriately based on the design, and (2) whether they claim effects for just the study participants or for “all” people. Eliminate choices that overgeneralize or that ignore the difference between association and causation.
Hints
Think about the design of the study
Focus on the phrase "randomly assigned." Does that describe an observational study or an experiment? How does that affect what kinds of conclusions you can draw?
Causal vs. non-causal language
Ask yourself: In this kind of study, are we allowed to say one thing "causes" another, or should we only say they are "associated"? Which choice matches that?
Scope of the conclusion
Look carefully for phrases like "for all people" versus "for the volunteers in this study." Based on how the participants were chosen, how broad a claim can we safely make?
Step-by-step Explanation
Identify the type of study
The volunteers were randomly assigned to receive either the supplement or a placebo. Random assignment of treatments makes this a randomized experiment, not just an observational study.
Understand what random assignment allows
In a randomized experiment, random assignment helps balance other factors (like age, stress, or lifestyle) between the groups. So if there is a clear difference in outcomes, we can reasonably say that the treatment caused the difference, at least for the people in the experiment.
Interpret “significantly more”
The phrase “average nightly sleep duration increased significantly more” in the supplement group means the difference between groups is unlikely to be due to random chance alone. This supports a real effect of the supplement compared with the placebo in this study.
Decide how far the conclusion can go
Even though we can make a causal statement, the study used volunteers, not a random sample of all people with chronic insomnia. That means we can be confident about cause-and-effect for the volunteers in this study, but we cannot be certain the same effect holds for all people with chronic insomnia. Therefore, the best conclusion is: The herbal supplement causes longer sleep for the volunteers in this study.