Before revision:   The first-stage regression is a natural framework to evaluate the validity of instruments . Thus, we are able to use the first-stage result and suggest testing procedures to evaluate the adequacy of instruments in IVQR models by evaluating their statistical significance .
After revision:  The first-stage regression is a natural framework to evaluate the validity of instruments , and in particular, the validity of the Jacobian identification conditions. Hence, we suggest testing procedures to evaluate the adequacy of instruments in IVQR models by evaluating their statistical significance .