in research, observer bias is a form of detection bias originating at a study's stage of observing or recording information. different observers may assess subjective criteria differently, and cognitive biases (including preconceptions and assumptions) can affect how a subject is assessed. for example, being aware of a subject's disease status may introduce a bias in how the outcome is assessed. observer bias can also occur when the subject knows they are being examined. when a subject knows they are being observed, it can cause them to act different to how they normally would, which could interfere with the experiment. another example examines police work, where police officers change their behavior based on who is watching.