1. Recall the assumption for the cmlrm var(;)=σ= const.i=1l,…,n (Homoskedasticity) 2. Counterexamples 1)rich family and poor family expenditures 2)large company and small company sales There exists heteroskedasticity in lots of econometric problems1. Recall the assumption for the CMLRM: (Homoskedasticity) 2. Counterexamples 1) rich family and poor family expenditures; 2) large company and small company sales. There exists heteroskedasticity in lots of econometric problems. 2 var( ) const. 1, , i  = = =i n