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# Open Online R Stream (https://www.wvbauer.com/doku.php/live_streams)
#
# By: Wolfgang Viechtbauer (https://www.wvbauer.com)
# Date: 2023-11-02
#
# Topic(s):
# - An Introduction to R
# https://cran.r-project.org/doc/manuals/r-release/R-intro.html
# - Section(s): 11.3 - 11.5
#
# last updated: 2023-11-08
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## 11.3 Generic functions for extracting model information
# inspect the mtcars dataset again
mtcars
# fit a linear regression model predicting the miles per gallon (mpg) of the
# cars from their horsepower (hp) and whether it is an automatic or manual
# transmission (am; 0 = automatic, 1 = manual)
res <- lm(mpg ~ hp + am, data=mtcars)
# print the 'res' object
res
# the above is the same as calling the print() function on the object
print(res)
# as we can see, printing the 'res' object is rather uninformative; all this
# gives us info about the model that was fitted and the model coefficients
# the lm() function returns an object of class 'lm'
class(res)
# this is a list
is.list(res)
# if we just want to see what is inside of this list object, we can remove the
# class and just print the raw list contents
unclass(res)
# to get more information about the fitted model, use summary()
summary(res)
# we can extract the coefficients with coef()
coef(res)
# so we could manually compute the predicted average gas mileage of cars with
# hp=100 and am=1 (i.e., with manual transmission) as follows
b <- coef(res)
b[[1]] + b[[2]]*100 + b[[3]]*1
# as we saw earlier, res has an element called 'coefficients' and this is
# really just what coef() is extracting from res
res$coefficients
# note: if possible, use 'extractor functions' like coef() to get at elements
# inside model objects; then you don't have to worry about how things are
# stored internally in such objects
# extract the variance-covariance matrix of the coefficients
vcov(res)
# the square-root of the diagonal elements of this matrix are the standard
# errors of the regression coefficients (as shown in summary(res))
sqrt(diag(vcov(res)))
# extract the residuals
resid(res)
# extract the standardized residuals
rstandard(res)
# extract the fitted values
fitted(res)
# plot of the fitted values versus residuals
plot(fitted(res), resid(res), pch=19)
abline(h=0, lty="dotted")
# we can also directly call plot() on the model object
par(mfrow=c(2,2))
plot(res)
par(mfrow=c(1,1))
# this gives four different plots:
#
# 1) plot of the fitted values versus residuals
# 2) qq-plot of the standardized residuals
# 3) plot of the fitted values versus the square-root of the absolute value of
# the standardized residuals
# 4) plot of the 'leverage' of the points versus the standardized residuals
#
# these different plots have different diagnostic purposes; to read a bit more
# about these plots (and others that can be created with plot() here), see:
help(plot.lm)
# above, we computed a predicted value manually; but we can use predict() to
# do the computations more conveniently
predict(res, newdata=data.frame(hp=100, am=1))
# with this, we can easily compute multiple predicted values simultaneously
predict(res, newdata=data.frame(hp=seq(60,200,by=20), am=1))
# look at the documentation of the predict method for lm objects
help(predict.lm)
# we see that we can get confidence intervals for the predictions by setting
# the 'interval' argument to "confidence" (with 95% CIs being the default)
predict(res, newdata=data.frame(hp=seq(60,200,by=20), am=1), interval="confidence")
# fit two models, one with just 'hp' as predictor and one with also 'am' as predictor
res0 <- lm(mpg ~ hp, data=mtcars)
res1 <- lm(mpg ~ hp + am, data=mtcars)
# model comparison (this is identical to testing the slope of 'am')
anova(res0, res1)
summary(res1)
# fit two models, one with just the intercept and one with both predictors
res0 <- lm(mpg ~ 1, data=mtcars)
res1 <- lm(mpg ~ hp + am, data=mtcars)
# sidenote: res0 is a model that doesn't take any of the characteristics of
# the cars into consideration when making a prediction about their gas
# mileage; or in other words, the estimated intercept of this model is equal
# to the average mpg value of the 32 cars in the dataset, and hence it
# predicts the same gas mileage for all 32 cars
# model comparison (this is identical to the omnibus F-test of the res1 model)
anova(res0, res1)
summary(res1)
# compare two more models, one with just 'hp' as predictor and the other one
# also with number of cylinders (as a factor / categorical predictor)
res0 <- lm(mpg ~ hp, data=mtcars)
res1 <- lm(mpg ~ hp + factor(cyl), data=mtcars)
anova(res0, res1)
# this is testing whether the cylinders factor as a whole is significant
# (while controlling for the horsepower of the cars)
# compare two more models, one assuming a linear relationship between 'hp' and
# gas mileage and one assuming a non-linear relationship (of the form of a
# cubic polynomial)
res0 <- lm(mpg ~ hp, data=mtcars)
res1 <- lm(mpg ~ poly(hp, degree=3), data=mtcars)
anova(res0, res1)
# plot the data and add the regression lines from the two models to the plot
plot(mpg ~ hp, data=mtcars, pch=21, bg="lightgray")
abline(res0, lwd=3)
hps <- 40:350
pred <- predict(res1, newdata=data.frame(hp=hps))
lines(hps, pred, lwd=3, col="red")
# treat hp as a categorical predictor
res2 <- lm(mpg ~ factor(hp), data=mtcars)
summary(res2)
# this model gives the best fit among all models that only take hp into
# consideration for making predictions about the gas mileage
# add the predicted values of this model to the plot
hps <- sort(unique(mtcars$hp))
pred <- predict(res2, newdata=data.frame(hp=hps))
lines(hps, pred, lwd=3, col="blue")
# add a legend
legend("topright", inset=.02, lty=1, col=c("black","red","blue"), lwd=3,
legend=c("linear model", "cubic polynomial model", "best fit model"))
# now test if this model gives a significantly better fit than the model that
# assumes that the relationship is linear
anova(res0, res1)
# this is sometimes called the 'lack of linearity test' or 'lack of fit test'
# (which goes back to Fisher, 1922; https://doi.org/10.2307/2341124)
# fit a model with multiple predictors
res <- lm(mpg ~ hp + am + factor(cyl) + wt, data=mtcars)
summary(res)
# when we use anova() just on a single model like the one above, then we get a
# sequence of tests, so testing if adding hp to the model improves the fit
# (yes), testing if adding am to the model when hp is already included
# improves the fit (yes), testing if adding factor(cyl) improves the fit when
# hp and am are already included (yes), and finally whether adding wt improves
# the fit when all other predictors are already included (also yes)
anova(res)
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## 11.4: Analysis of variance and model comparison
# a simple example of aov(): one-way analysis of variance (ANOVA)
res <- aov(mpg ~ factor(cyl), data=mtcars)
summary(res)
# using aov(), one can also fit more complex ANOVA-type models (e.g., with
# multiple between- and/or within-subject factors); depending on the model,
# this requires the use of Error() in the model formula and can get more
# complex; a nice package that can simplify the fitting of such models is the
# 'ez' package: https://cran.r-project.org/package=ez
# sidenote: we'll skip some of the other points being raised here
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## 11.5: Updating fitted models
# fit a model
res1 <- lm(mpg ~ hp + am, data=mtcars)
summary(res1)
# update the model by adding some additional predictors
res2 <- update(res1, . ~ . + factor(cyl) + wt)
summary(res2)
# note: the period (.) in update() means: 'what was here before'
# update the model by removing some predictor
res3 <- update(res2, . ~ . - factor(cyl))
summary(res3)
# careful: outside of update(), the period in a formula has a different
# meaning, typically to stand for all variables (except for the outcome)
res <- lm(mpg ~ ., data=mtcars)
summary(res)
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