normcheck: Testing for normality plot

In s20x: Functions for University of Auckland Course STATS 201/208 Data Analysis

Testing for normality plot

Description

Plots two plots side by side. First, it draws a normal Q-Q plot of the residuals, along with a line with intercept equal to the mean of the residuals and slope equal to the standard deviation of the residuals. If shapiro.wilk = TRUE, the P-value from the Shapiro-Wilk test for normality is shown in the top-left corner of the Q-Q plot. Second, it draws a histogram of the residuals. A normal distribution is fitted and superimposed over the histogram. Note: if you want to leave the x-axis blank in the histogram then use xlab = c("Theoretical Quantiles", " ") , i.e. leave a space between the quotes. If you do not leave a space, information will be extracted from x.

Usage

normcheck(x, ...)

## Default S3 method:
normcheck(
  x,
  xlab = c("Theoretical Quantiles", ""),
  ylab = c("Sample Quantiles", ""),
  main = c("", ""),
  col = "light blue",
  bootstrap = FALSE,
  B = 5,
  bpch = 3,
  bcol = "lightgrey",
  shapiro.wilk = FALSE,
  whichPlot = 1:2,
  usePar = TRUE,
  engine = c("base", "ggplot2"),
  ...
)

## S3 method for class 'lm'
normcheck(
  x,
  xlab = c("Theoretical Quantiles", ""),
  ylab = c("Sample Quantiles", ""),
  main = c("", ""),
  col = "light blue",
  bootstrap = FALSE,
  B = 5,
  bpch = 3,
  bcol = "lightgrey",
  shapiro.wilk = FALSE,
  whichPlot = 1:2,
  usePar = TRUE,
  engine = c("base", "ggplot2"),
  ...
)

## S3 method for class 'tslm'
normcheck(
  x,
  xlab = c("Theoretical Quantiles", ""),
  ylab = c("Sample Quantiles", ""),
  main = c("", ""),
  col = "light blue",
  bootstrap = FALSE,
  B = 5,
  bpch = 3,
  bcol = "lightgrey",
  shapiro.wilk = FALSE,
  whichPlot = 1:2,
  usePar = TRUE,
  residualType = "normalised",
  engine = c("base", "ggplot2"),
  ...
)

Arguments

x	the residuals from fitting a linear model. Alternatively, a fitted lm object.
...	additional arguments which are passed to both qqnorm and hist for the base engine. Extra arguments are currently ignored by the ggplot2 engine.
xlab	a title for the x-axis of both the Q-Q plot and the histogram: see title.
ylab	a title for the y-axis of both the Q-Q plot and the histogram: see title.
main	a title for both the Q-Q plot and the histogram: see title.
col	a colour for the bars of the histogram.
bootstrap	if TRUE then B samples will be taken from a Normal distribution with the same mean and standard deviation as x. These will be plotted in a lighter colour behind the empirical quantiles to show how much variation would be expected in the Q-Q plot for a sample of the same size from a truly normal distribution.
B	the number of bootstrap samples to take. Five should usually be sufficient.
bpch	the plotting symbol used for the bootstrap samples. Legal values are the same as any legal value for pch as defined in par.
bcol	the plotting colour used for the bootstrap samples. Legal values are the same as any legal value for col as defined in par.
shapiro.wilk	if TRUE, the P-value from the Shapiro-Wilk test for normality is displayed in the top-left corner of the Q-Q plot.
whichPlot	legal values are 1, 2, and any pair of the two, i.e. 1:2, 2:1, c(1,2), c(2,1), or variants of c(1,1). 1:2 is used by default and draws a normal Q-Q plot and a histogram of the residuals in that order. The order of the labels in xlab and ylab assume this order, and will be reordered automatically if the order is anything other than 1:2.
usePar	if TRUE, this function sets par for the user. If FALSE, this function assumes par has been set by the user and should not be overridden. Ignored by the ggplot2 engine.
engine	plotting engine to use. The default, "base", preserves the original base graphics output. Use "ggplot2" for optional ggplot2 objects.
residualType	for tslm objects, the residual scale to use in the normality plots. The default is "normalised", which checks the residuals after accounting for the fitted error correlation structure. "normalised" and "normalized" are both accepted for compatibility. Other choices are "response" and "pearson".

Details

The default base graphics engine preserves the original teaching plots and draws directly on the active graphics device. The optional ggplot2 engine is intended for users who want reusable plot objects for reports or further customisation; it requires ggplot2 to be installed and returns ggplot objects instead of drawing base graphics side effects.

Value

Draws the selected normality diagnostic plots when using the base engine. With engine = "ggplot2", returns a ggplot object for a single selected plot or a named list of ggplot objects for multiple selected plots. When multiple ggplot2 plots are selected, printing the returned object draws the plots side by side to match the base graphics teaching layout.

See Also

shapiro.test.

Examples

# Synthetic teaching example: an exponential growth curve
set.seed(123)
e = rnorm(100, 0, 0.1)
x = rnorm(100)
y = exp(5 + 3 * x + e)
fit = lm(y ~ x)
normcheck(fit)

# An exponential growth curve with the correct transformation
fit = lm(log(y) ~ x)
normcheck(fit)

# Same example as above except we use normcheck.default
normcheck(residuals(fit))

# Peruvian Indians data
data(peru.df)
peruFit = lm(BP ~ weight, data = peru.df)
normcheck(peruFit)

# Optional ggplot2 engine for reusable plot objects
if (requireNamespace("ggplot2", quietly = TRUE)) {
  normPlots = normcheck(peruFit, engine = "ggplot2")
  names(normPlots)

  normcheck(peruFit, engine = "ggplot2", whichPlot = 1)
  normcheck(peruFit, engine = "ggplot2", whichPlot = 2)
}

s20x documentation built on July 1, 2026, 9:06 a.m.