Observer Bias

What is observer bias?

Observer bias occurs when an assessor's expectations or knowledge affect how observations are measured, classified or interpreted. For example, knowledge of treatment assignment could influence a subjective outcome rating.

Hróbjartsson and colleagues (2012) compared blinded and nonblinded assessments within clinical trials. On average, nonblinded assessors produced more favorable treatment estimates, although the results varied. Substantial agreement between assessors did not rule out bias.

Blinding assessors to information they do not need, specifying measurement rules in advance, and documenting disagreements can help address this risk. Awareness and multiple observers alone do not guarantee unbiased measurement, especially if observers share the same expectations.

Observer bias vs. observer effect

Observer bias concerns how an observation is assessed or recorded. People changing their behavior because they know they are watched is a different issue: reactivity. Both can affect naturalistic observation. Researcher expectations may also influence participants; distinguishing these routes helps identify what a study needs to control.