Name a key threat to internal validity related to participants changing over time during a study.

• A. History.
• B. Maturation.
• C. Instrumentation.
• D. Selection bias.

Answer: (B)

Changes within participants as time passes can alter the outcome independent of any treatment. This is maturation. People naturally develop, age, fatigue, or become more familiar with the testing situation over the course of a study, and these changes can make results look like an effect of the intervention when they are really just part of growth or weariness. For example, in a long study with children, reading ability might improve over months due to natural development rather than the program being tested.

Other threats explain different problems: history would involve external events happening between measurements that affect all participants, instrumentation would involve changes in how the outcome is measured, and selection bias refers to preexisting differences between groups at the start of the study.