Your Textbook Derives the OLS Estimator As Show That the Estimator Does Not Exist If There Are

Essay

Your textbook derives the OLS estimator as $\hat { \boldsymbol { \beta } } = \left( \boldsymbol { X } ^ { \prime } \boldsymbol { X } \right) ^ { - 1 } \boldsymbol { X } ^ { \prime } \boldsymbol { Y }$ Show that the estimator does not exist if there are fewer observations than the number of
explanatory variables, including the constant.What is the rank of X′X in this case?