# unbiasedness of parameters of simple linear regression model

Whether the parameters of simple linear regression model beta0(intercept) and beta1(slope) are unbiased? I was taught that these are unbiased estimators in my college. But one of my friends is telling that they are not unbiased. I will be grateful if I get a proper solution

• If the assumptions are met, they are indeed unbiased. So neither yourself nor your friend is wrong. – SmallChess May 9 '17 at 0:32
• @SmallChess That seems like an answer, if you add a little detail. – Peter Flom - Reinstate Monica May 9 '17 at 10:46

Assume there is non-zero correlation between $X_1$ and $X_2$. Assume your true model is $Y = b_1X1 + b_2X2 + e$ in a standard OLS regression framework.
If you miss out the important $X_2$ predictor, the residuals are forced to "eat" the missing effects from $X_2$. In statistics, we say the residuals are no longer uncorrelated to $X_1$.