Estimator variables are external factors that influence eyewitness testimony and how accurate the recollection of the events are. Estimator variables cannot be controlled by others.
Examples are how good the witness' vision is, how close the witness was to the event, and how clear the weather and visual conditions were on that day. These variables are influential to the testimony of the witness and cannot be controlled by legal professionals. Opposed to this concept are system variables which can be controlled by individuals. Examples of system variables include how and when a police lineup is arranged and instructions given to the witness.