2. Inclusion of irrelevant variables Overfitting a Model Inclusion of irrelevant variables will certainly increase R2, which might increase the predictive power of the model True mode:Y=B1+B2X2i+u (13. 9) Misspecified model Y=A1+A2×2+A3X3+v;(13.10)Inclusion of irrelevant variables will certainly increase R2 ,which might increase the predictive power of the model. True model: Yi=B1+B2X2i+μi （13.9） Misspecified model: Yi=A1+A2X2i+ A3X3i+ vi （13.10） 2.Inclusion of Irrelevant Variables: —— “Overfitting” a Model