Suppose the true model is $y_i=\beta_0+\beta_1x_{1i}+\beta_2x_{2i}+\epsilon_i$, but $x_2$ is not observable.
If we run a regression instead on the model $y_i=\beta_0+\beta_1x_{1i}+\eta_{1i}$, what is an expression for the bias in the coefficient $\hat{\beta_1}$ alone?
