Which term best describes a threat to internal validity arising from outside events unrelated to manipulation?

• A. History effects
• B. Maturation
• C. Attrition
• D. Instrumentation

Answer: (A)

External events during a study that are not caused by the manipulation can shift outcomes, creating a history threat to internal validity. History effects occur when something outside the experimental manipulation happens between measurements and affects participants’ responses. This makes it difficult to know whether observed changes come from the treatment or the external event. For example, a major news event during the study could alter attitudes toward the topic regardless of the intervention. By contrast, maturation refers to natural changes within participants over time, attrition to differential dropout, and instrumentation to changes in measurement tools or procedures. Therefore, the term that best describes this threat is history effects.