# qq.plot

##### 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

```
qq.plot(x, ...)
qqp(...)
## S3 method for class 'default':
qq.plot(x, distribution="norm", ylab=deparse(substitute(x)),
xlab=paste(distribution, "quantiles"), main=NULL, las=par("las"),
envelope=.95, labels=FALSE, col=palette()[2], lwd=2, pch=1, cex=1,
line=c("quartiles", "robust", "none"), ...)
## S3 method for class 'lm':
qq.plot(x, main=NULL, xlab=paste(distribution, "Quantiles"),
ylab=paste("Studentized Residuals(", deparse(substitute(x)), ")",
sep = ""),
distribution=c("t", "norm"), line=c("quartiles", "robust", "none"),
las=par('las'), simulate=FALSE, envelope=0.95, labels=names(rstudent),
reps=100, col=palette()[2], lwd=2, pch=1, cex=1, ...)
```

##### 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. - labels
- vector of point labels for interactive point identification,
or
`FALSE`

for no labels. - las
- if
`0`

, ticks labels are drawn parallel to the axis; set to`1`

for horizontal labels (see`par`

). - col
- color for points and lines; the default is the
*second*entry in the current color palette (see`palette`

and`par`

). - 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`

. - lwd
- line width; default is
`2`

(see`par`

). Confidence envelopes are drawn at half this line width. - 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.

##### 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 are plotted against the
appropriate t-distribution.
The function `qqp`

is an abbreviation for `qq.plot`

.

##### Value

`NULL`

. These functions are used only for their side effect (to make a graph).

##### References

Fox, J. (1997)
*Applied Regression, Linear Models, and Related Methods.* Sage.
Atkinson, A. C. (1985)
*Plots, Transformations, and Regression.* Oxford.

##### See Also

##### Examples

```
x<-rchisq(100, df=2)
qq.plot(x)
qq.plot(x, dist="chisq", df=2)
qq.plot(lm(interlocks~assets+sector+nation, data=Ornstein), sim=TRUE)
```

*Documentation reproduced from package car, version 1.2-2, License: GPL version 2 or newer*