Which term refers to unintended changes in the way an observer uses a measurement system over the course of an investigation, resulting in measurement error?

• A. Observer drift
• B. Observer reactivity
• C. Measurement bias
• D. Interobserver agreement

Answer: (A)

Observer drift refers to unintended changes in how the measurement system is used as the investigation progresses, which introduces systematic measurement error. Over time, an observer might apply recording rules slightly differently, interpret event boundaries in a shifting way, or become less consistent in data collection. This drift can bias data because the error comes from the observer’s changing method rather than from the behavior being measured. It differs from observer reactivity, where the subject alters behavior due to being observed, and from interobserver agreement, which concerns consistency between multiple observers at the same time. Preventing drift involves clear, objective definitions, ongoing training and calibration, and regular reliability checks to ensure the measurement system is applied consistently throughout the study.