The term observer effect generally refers to the possibility that an act of observation may affect the properties of what is observed. However, depending on the context and the mechanisms involved, it may indicate effects of a very different nature. Observer effects are a threat to validity in much of educational research. After providing examples of observer effects to illustrate its meaning, this entry examines how to limit observer effect. The entry also considers common misconceptions about observer effect.

Imagine measuring the pressure of an automobile tire. When connecting the manometer, it is easy to let out some air, so that the measured pressure may not correspond to the pressure of the tire before the act of the measurement was initiated. Similarly, imagine that we want ...