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Using R for Introductory Statistics : John Verzani

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<strong>Using</strong> R <strong>for</strong> introductory statistics 326Is there an indication of an interaction? Do both the number of cylinders and the typeof transmission make a difference?11.28 The data set ToothGrowth has measurements of tooth growth (len) of guineapigs <strong>for</strong> different dosages of Vitamin C (dose) and two different delivery methods (supp).Per<strong>for</strong>m a two-way analysis of variance of tooth growth modeled by dosage anddelivery method. First, fit the full model including interactions and use the F-test tocompare this with the additive model.11.29 The data set OrchardSprays contains measurements on the effectiveness ofvarious sprays on repelling honeybees. The variable decrease measures effectiveness ofthe spray, treatment records the type of treatment, and rowpos records the row in the fieldthe measurement comes from.Make an interaction plot of the mean of decrease with treatment as a trace factor. Thenfit the additive analysis-of-variance model and the model with interaction. Compare thetwo models using anova(). Is the interaction model suggested by the results of themodeling?

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