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Lecture 10 Feb 1 2006

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Added-variable Added variable plots give you a visual sense of whether x2 is a useful addition to the model: E(y|x1) = a + b x1

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Steps to making one Regress y on x1 Compute residuals of y on x1: y "-"x1 (remove x1 from y) Regress x2 on x1 Compute residuals of x1 on x2: x2 "-"x1 (remove x1 from x2) Plot y"-"x1 vs. x2 "-"x1

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Interpret If there is a "significant" slope, then x2 is useful. Slope of the added variable plot is the same as the coefficient if you fit E(y|x1, x2) = B 0 + B 1 x1 + B 2 x2

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Significance Tests Find the slope of an added variable plot, and do a t-test to see if the slope is significant. The value of the t-stat is almost the same as the value of the t-stat for the "full" model. The p-values will differ because the degrees of freedom are different: n-2 for added- variable slope, n-3 for full model

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Mussel Beds

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Is density related to food levels?

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Is density related to human use?

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If we know the human use level, do we need to know food leve?

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Added-Variable

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Summary of added variable plot slope = 10.87 additional amount of food worth 10.87 mm of thickness, on avg., controlling for human use t = 4.742, p=1.45e-05

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Summary of lm(thickness~food+human.use) E(thickness|food, human) = 63+18.87*food - 6.294 * human.use t_food + 4.70, p = 1.84e-05

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Testing one variable: Plan #1 H0: Beta1 = 0, Beta0, Beta2, Beta3, etc. "arbitrary" Ha: Beta1 <> 0, others arbitrary Fit full model: y = B0+B1*x1+B2*x2+B3*x3 etc. Fit reduced model y = B0 + B2*x2+B3*x3 Compare RSS

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partial F-test Compare RSS (full) with RSS (reduced) Note: RSS(reduced) will be ???? than RSS(full) F = ( ( RSS(red) - RSS(full) ) /1 ) /RSS(full)/(n-p) Note: The denominator is just = ????

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Formula for partial F (Reduction in RSS)/DF num divided by RSS/DF den DF of a model is n - # of parameters estimated. DF num is DF(full)-DF(reduced) DF den is n - p

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In R (long) full <- lm(y ~ x1 + x2 + x3 + x4) red <- lm(y~x2 + x3 + x4) anova(full); anova(red) compute by hand from output

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R(short) full <- lm(y ~ x1+x2+x3) anova(full) Read output

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Plan #2 t^2 = F full <- lm(y~x1+x2+x3) summary(full) look at t-statistic

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