Name three threats to internal validity in experimental designs.

• A. History, maturation, and instrumentation
• B. Randomization, placebo effects, and demand characteristics
• C. Selection bias, attrition, and regression to the mean
• D. Contamination, maturation, and resonance

Answer: (A)

Threats to internal validity arise when something other than the experimental manipulation influences the outcome. History, maturation, and instrumentation are classic examples because each can mimic or obscure a treatment effect. History means events outside the study occur between measurements and affect results, making it look like the treatment caused change when it was actually due to those external events. Maturation refers to natural changes in participants over time—age, fatigue, practice effects—that can shift the dependent variable regardless of the intervention. Instrumentation involves changes in the measurement process or the observers’ scoring over time, which can produce differences in results that are not due to the treatment itself. These three collectively capture how outcomes can be confounded by time-related or measurement-related factors, undermining causal inferences. Other options mix elements that aren’t classic internal validity threats in this trio—for example, randomization is a design tool to reduce bias, while placebo effects and demand characteristics relate to expectations and measurement issues rather than external events, development, or instrument changes.