# Partial F-tests and Lack-of-Fit Tests

# PARTIAL F-TEST.

# Consider full and reduced models

> setwd("C:\Users\baron\624 Data Analysis\data")

> load("Auto.rda")

> attach(Auto)

> reg_full = lm(mpg ~ year + acceleration + horsepower + weight)

# How to test significance of year and acceleration?

> reg_reduced = lm(mpg ~ horsepower + weight)

> anova( reg_full, reg_reduced )

Analysis of Variance Table

Model 1: mpg ~ year + acceleration + horsepower + weight

Model 2: mpg ~ horsepower + weight

Res.Df RSS Df Sum of Sq F Pr(>F)

1 387 4558.0

2 389 6993.8 -2 -2435.8 103.41 < 2.2e-16 ***

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Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

# The p-value comparing these two models is very significant, so the two variables make a significant contribution for the prediction of mpg, in addition of weight and horsepower.

# LACK-OF-FIT.

# Here we test linearity by comparing the linear model (reduced) with the model with dummy variables, one for each value of X (full model that does not assume linearity).

> reg_reduced = lm(mpg ~ cylinders)

> reg_full = lm(mpg ~ as.factor(cylinders))

> anova( reg_full, reg_reduced )

Analysis of Variance Table

Model 1: mpg ~ as.factor(cylinders)

Model 2: mpg ~ cylinders

Res.Df RSS Df Sum of Sq F Pr(>F)

1 387 8544.5

2 390 9415.9 -3 -871.42 13.156 3.383e-08 ***

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Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

# Low p-value shows that the relation of mpg to the number of cylinders is non-linear.

# Continuous case – what to do if the X-variable has no repeated values?

> round(horsepower/10)*10

[1] 130 160 150 150 140 200 220 220 220 190 170 160 150 220 100 100 100 80

[19] 90 50 90 90 100 110 90 220 200 210 190 90 90 100 100 100 100 90

< truncated >

> reg_reduced = lm(mpg ~ horsepower)

> hp_rounded = round(horsepower/10)*10

> reg_full = lm( mpg ~ as.factor(hp_rounded) )

> anova( reg_full, reg_reduced )

Analysis of Variance Table

Model 1: mpg ~ as.factor(hp_rounded)

Model 2: mpg ~ horsepower

Res.Df RSS Df Sum of Sq F Pr(>F)

1 373 7101.9

2 390 9385.9 -17 -2284 7.0565 6.662e-15 ***

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Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

# The full model is significantly better. So, mpg is a non-linear function of horsepower.