The Conditions for a Valid Instruments Do Not Include the Following

Short Answer

The conditions for a valid instruments do not include the following: a. each instrument must be uncorrelated with the error term.
b. each one of the instrumental variables must be normally distributed.
c. at least one of the instruments must enter the population regression of $X$ on the $Z$ 's and the $W$ 's.
d. perfect multicollinearity between the predicted endogenous variables and the exogenous variables must be ruled out.