Rank(X'X)=k, Full Rank

endogeneity ass.

 

Rank(X)=k, Full Column Rank

X's size: n x k

 

n: # of observations

k: # of explanatory variables

 

It's natural to suppose n is larger than k.

 

Rank(X'X)=Rank(X)

 

Therefore, (X'X)^-1 exists.