Hi,

I am doing Durbin–Wu–Hausman test to test my independent variables for endogeniety.

My data is cross sectional and I have a matched pairs design. For the purpose of analysis, I would use either logistic regression or probit or conditional logistic regression.

I am using the following steps to test for endogeneity:

1) regress X1 X2 X3 X4 X5 X6 X7 (X1 to X7 are all independent variables and X1 is being tested for endogeneity)

2) predict X1_res, res

3) regress DV X1 X2 X3 X4 X5 X6 X7 X1_res (DV is the dependent variable)

4) test X1_res

For a lot of the independent variables, STATA is returning the message that the constraint is dropped. For eg: o.CDualy_res = 0
Constraint 1 dropped

Could you please help me understand why are the constraints is getting dropped?