What error occurs when the definition of the target behavior deviates or changes over time?

• A. Observer Fatigue
• B. Reactivity
• C. Observer Drift
• D. Observer Bias

Answer: (C)

Observer drift occurs when the definition of the target behavior changes over time or is applied differently by observers. When the criteria shift, what gets counted can vary from session to session, making data unreliable and comparisons across time invalid. This undermines the ability to track true change or evaluate intervention effects. To prevent drift, use a precise, observable definition and conduct ongoing observer calibration to ensure consistent application across time. The other issues—observer fatigue, which reduces attention; reactivity, where being observed alters behavior; and observer bias, where expectations influence scoring—address separate problems in measurement but do not capture the problem of a shifting or inconsistently applied definition.