Which of the following is a threat to internal validity?

• A. History
• B. Maturation
• C. Testing
• D. Instrumentation

Answer: (D)

Internal validity is compromised when the way outcomes are measured itself changes across conditions or over time. Instrumentation is the threat that the measurement process—tools, scoring procedures, or observers—changes in a way that affects the results. If the instrument or the way you collect data differs from one time point to another, observed differences may reflect those measurement changes rather than any real effect of the intervention. For example, using a different survey form, adjusting scoring rules, or having observers drift in how they rate responses can create artificial changes in outcomes.

To guard against this, keep measurement methods constant, train data collectors consistently, use the same instrumentation, and check reliability regularly. History, maturation, and testing are also threats to internal validity, but they involve external events, natural participant changes, or retesting effects rather than changes in how outcomes are measured.