Which design feature helps determine the causal effect of the WM training in the Klinberg study?

• A. Observational data
• B. Large sample size only
• C. Use of a control group
• D. Short duration

Answer: (C)

Isolating cause and effect comes from comparing what happens with the training to what happens without it. A control group provides that essential comparison: both groups are treated the same in every way except for receiving the working memory training. Any difference in outcomes can then be more confidently attributed to the training itself rather than to other factors like test familiarity, practice effects, or external events. This setup is what makes causal claims about the training possible.

Observational data, in contrast, can show associations but not prove that the training caused any change, because people aren’t randomly assigned and other variables could drive the results. A large sample size helps precision and reliability but doesn’t by itself establish that the training caused changes. A short duration may limit what you can observe and affect generalizability, but it doesn’t determine whether the training is causing the effects.