Say I want to measure weight as a function of gender and individual over time.
Something I dont quite understand is why a fixed effect model for individuals always block out gender?
What is the proof that it will be perfectly collinear?
From my understanding most fixed effects models introduce individual dummies, but I dont see how this will generate perfect collinearity.
A fixed effects model can be regarded as a regression with a dummy variable for each group. This dummy variable is time invariant. If you have another variable which is time invariant for a group it is a multiple of the dummy for that group and is thus perfectly colinear with that dummu.