Which threat is a classic internal validity threat due to participants changing over time independent of the intervention?

• A. Maturation
• B. Selection bias
• C. Instrument decay
• D. Experimental mortality

Answer: (A)

Maturation is a classic internal validity threat because participants naturally change over time, independent of any intervention. As people develop, learn, age, or become more familiar with testing, their outcomes can shift merely due to passage of time. If you’re measuring effects before and after an intervention, these natural changes can look like the intervention effects when they’re really due to growth or development. That’s why maturation threatens internal validity: it provides an alternative explanation for observed change that isn’t caused by the treatment.

Selection bias involves differences between groups at the start, instrument decay is about the measurement tool changing in reliability over time, and experimental mortality (attrition) is about differential dropout. While each is a validity concern, maturation specifically centers on changes within participants themselves over time unrelated to the intervention.