Variation, observer means: Variation in observation could be caused by the observer missing an important measurement or making an error.
(in Medical Dictionary)

What else does Variation, observer mean?

Variation by observer: Inaccuracy in measuring, which can lead to error.
(in Medical Dictionary)

Inter-observer variability is the variation in the results of two or more observers who examine the same material. Inter-observer variation refers to the variation experienced by one observer when they examine the same material multiple times.
(in Medical Dictionary)