This common phenomenon is not necessarily either observer error or bias. There are two varieties. Interobserver variation occurs when two or more observers observe the same phenomenon or situation and arrive at different conclusions. Examples include grading essay-type examination papers; assessing psychological symptoms; palpating tumors; and interpreting x-rays, notably chest x-rays where varying interpretation has been studied in detail, blood films, electrocardiograms, and almost every other kind of observational assessment in clinical medicine. Commonly no two experts agree completely on what they observe and how to interpret it. Intraobserver variation occurs when the same observer repeats an observation, measurement, or assessment, especially of a complex phenomenon that is observed again after an interval during which other unrelated observations have been made or other tasks performed. The subsequent observation may lead to a conclusion that differs from the original one.
Subjects: Public Health and Epidemiology.