A weak instrument is an instrumental variable:
A) whose distribution is non-normal.
B) whose partial correlation with the outcome is small.
C) whose partial correlation with the (endogenous) treatment variable is small.
D) that suffers from heteroscedasticity.

Question

Quizplus · Accepted Answer

A weak instrument is one whose partial correlation with the endogenous treatment variable is small, meaning that it explains very little of the variation in the treatment variable. This makes it difficult to use the instrument to identify the causal effect of the treatment on the outcome. The other choices are not necessarily indicative of a weak instrument.

A Weak Instrument Is an Instrumental Variable