Instrumental Variable

Unlike an observed control variable, an instrumental variable is assumed not to have any direct effect on the outcome.

Instead, the instrumental variable is thought to influence only the selection into the treatment condition.

Example

Source: https://largescaleassessmentsineducation.springeropen.com/articles/10.1186/s40536-016-0018-2

See Also: https://en.wikipedia.org/wiki/Instrumental_variables_estimation