Identification
In the instrumental variable regression, if we have multiple endogenous regressors and multiple instruments the coefficients on the endogenous regressors are said to be:
Exactly identified if m = k.
Overidentified if m > k.
Underidentified if m < k.
The parameters are not identified if there are fewer instruments than there are covariates or, equivalently, if there are fewer excluded instruments than there are endogenous covariates in the equation of interest.
Read more about this topic: Instrumental Variable
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