qqPlot

0th

Percentile

Quantile-Comparison Plots

Plots empirical quantiles of a variable, or of studentized residuals from a linear model, against theoretical quantiles of a comparison distribution.

Keywords
distribution, regression, univar
Usage
qqPlot(x, ...)

qqp(...)

## S3 method for class 'default':
qqPlot(x, distribution="norm", ...,
	ylab=deparse(substitute(x)), xlab=paste(distribution, "quantiles"), 
	main=NULL, las=par("las"),
	envelope=.95,  
	col=palette()[1], col.lines=palette()[2], lwd=2, pch=1, cex=par("cex"), 
	line=c("quartiles", "robust", "none"),  
	labels = if(!is.null(names(x))) names(x) else seq(along=x),
	id.method = "y", 
	id.n =if(id.method[1]=="identify") Inf else 0,
	id.cex=1, id.col=palette()[1], grid=TRUE)

## S3 method for class 'lm':
qqPlot(x, xlab=paste(distribution, "Quantiles"),
	ylab=paste("Studentized Residuals(", deparse(substitute(x)), ")",
		sep=""), main=NULL,
	distribution=c("t", "norm"), line=c("robust", "quartiles", "none"),
	las=par("las"), simulate=TRUE, envelope=.95,  
	reps=100, col=palette()[1], col.lines=palette()[2], lwd=2, 
	pch=1, cex=par("cex"),
	labels, id.method = "y", 
	id.n = if(id.method[1]=="identify") Inf else 0,
	id.cex=1, id.col=palette()[1], grid=TRUE, ...)
Arguments
x
vector of numeric values or lm object.
distribution
root name of comparison distribution -- e.g., "norm" for the normal distribution; t for the t-distribution.
ylab
label for vertical (empirical quantiles) axis.
xlab
label for horizontal (comparison quantiles) axis.
main
label for plot.
envelope
confidence level for point-wise confidence envelope, or FALSE for no envelope.
las
if 0, ticks labels are drawn parallel to the axis; set to 1 for horizontal labels (see par).
col
color for points; the default is the first entry in the current color palette (see palette and par).
col.lines
color for lines; the default is the second entry in the current color palette.
pch
plotting character for points; default is 1 (a circle, see par).
cex
factor for expanding the size of plotted symbols; the default is 1.
labels
vector of text strings to be used to identify points, defaults to names(x) or observation numbers if names(x) is NULL.
id.method
point identification method. The default id.method="y" will identify the id.n points with the largest value of abs(y-mean(y)). See showLabels for othe
id.n
number of points labeled. If id.n=0, the default, no point identification.
id.cex
set size of the text for point labels; the default is cex (which is typically 1).
id.col
color for the point labels.
lwd
line width; default is 2 (see par).
line
"quartiles" to pass a line through the quartile-pairs, or "robust" for a robust-regression line; the latter uses the rlm function in the MASS package. Specifying line = "none" supp
simulate
if TRUE calculate confidence envelope by parametric bootstrap; for lm object only. The method is due to Atkinson (1985).
reps
integer; number of bootstrap replications for confidence envelope.
...
arguments such as df to be passed to the appropriate quantile function.
grid
If TRUE, the default, a light-gray background grid is put on the graph
Details

Draws theoretical quantile-comparison plots for variables and for studentized residuals from a linear model. A comparison line is drawn on the plot either through the quartiles of the two distributions, or by robust regression. Any distribution for which quantile and density functions exist in R (with prefixes q and d, respectively) may be used. Studentized residuals from linear models are plotted against the appropriate t-distribution. The function qqp is an abbreviation for qqPlot.

Value

  • These functions return the labels of identified points.

References

Fox, J. (2008) Applied Regression Analysis and Generalized Linear Models, Second Edition. Sage. Fox, J. and Weisberg, S. (2011) An R Companion to Applied Regression, Second Edition, Sage. Atkinson, A. C. (1985) Plots, Transformations, and Regression. Oxford.

See Also

qqplot, qqnorm, qqline, showLabels

Aliases
  • qqPlot
  • qqp
  • qqPlot.default
  • qqPlot.lm
  • qqPlot.glm
Examples
x<-rchisq(100, df=2)
qqPlot(x)
qqPlot(x, dist="chisq", df=2)

qqPlot(lm(prestige ~ income + education + type, data=Duncan), 
	envelope=.99)
Documentation reproduced from package car, version 2.0-13, License: GPL (>= 2)

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