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.

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#### Best Answer

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.