Which of the following can cause poor internal validity?

Which of the following can cause poor internal validity?

• A. Confounding variables
• B. Randomisation
• C. Large sample size
• D. Blinding

Answer: (A)

Confounding variables threaten internal validity because they are extraneous factors linked to both the treatment and the outcome, creating alternative explanations for observed effects. If a confounder is present and not controlled, you can’t be sure whether changes in the outcome are due to the intervention or to the confounder. For example, if more motivated participants end up in the treatment group, their improved outcomes might reflect motivation rather than the treatment itself. Randomisation helps prevent this by evenly distributing such factors across groups, so differences in outcome are more likely due to the intervention. A large sample size reduces random error and improves precision but does not by itself protect against confounding. Blinding reduces bias in measurement and expectations, also protecting internal validity rather than causing problems. So confounding variables are the factor that can cause poor internal validity.

Confounding variables threaten internal validity because they are extraneous factors linked to both the treatment and the outcome, creating alternative explanations for observed effects. If a confounder is present and not controlled, you can’t be sure whether changes in the outcome are due to the intervention or to the confounder. For example, if more motivated participants end up in the treatment group, their improved outcomes might reflect motivation rather than the treatment itself. Randomisation helps prevent this by evenly distributing such factors across groups, so differences in outcome are more likely due to the intervention. A large sample size reduces random error and improves precision but does not by itself protect against confounding. Blinding reduces bias in measurement and expectations, also protecting internal validity rather than causing problems. So confounding variables are the factor that can cause poor internal validity.