Please, can someone help me in the group, I am doing research in the area of health economics (child nutrition). I have run a fixed effect, random effect and Hausman test suggested that the fixed-effect model is accepted. The result makes sense because it confirms that there is an unobserved individual-specific effect that is associated with the regressor. Now, I want to do a sensitivity test, I have tried IV and control function analysis but the results come out to be negative. Can someone help me with a suggestion of methodology that I can use for robustness check or sensitivity test? ​​​​​​​