4-22-09

# 4-22-09 - implies MORE explanatory power • Simultaneously...

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Economic Statistics – Class Notes – 4/22/09 Q: How can we tell if the addition of new explanatory variables really improves the model. Model == a model of supply and demand for eggs Income affects demand, so you have to have a model for income as well 10. Adjusted R^2 (NOT regular R^2) a.) Recall: a. SST = SSE + SSR b.) Therefore, R^2 can never decrease as additional explanatory variables are added. R^2 tends to rise with more explanatory variables (Xi’s) c.) So how can we tell if the extra Xi’s (explanatory variables) really improve the model? ANSWER: Use an adjusted R^2.

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Recall: So, as we add extra variables, SSE falls and adjusted R^2 rises, which
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Unformatted text preview: implies MORE explanatory power. • Simultaneously, as more variables are added, “m” rises,” which causes adjusted R^2 to fall, which implies less explanatory. • Hence, if the SSE effect dominates the “m effect,” adjusted R^2 rises and the model has more explanatory power. o BUT you don’t want to just throw in extraneous variables into your model to increase adj. R^2 because economic theory prevents it. If you only have one model, there is NO need for adjusted R^2 just use regular R^2...
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4-22-09 - implies MORE explanatory power • Simultaneously...

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