# ancova

From HH v3.1-25
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##### Compute and plot oneway analysis of covariance

Compute and plot oneway analysis of covariance. The result object is an ancova object which consists of an ordinary aov object with an additional trellis attribute. The trellis attribute is a trellis object consisting of a series of plots of y ~ x. The left set of panels is conditioned on the levels of the factor groups. The right panel is a superpose of all the groups.

Keywords
models, hplot, regression, dplot
##### Usage
ancova(formula, data.in = NULL, ...,
x, groups, transpose = FALSE,
display.plot.command = FALSE,
superpose.level.name = "superpose",
ignore.groups = FALSE, ignore.groups.name = "ignore.groups",
blocks, blocks.pch = letters[seq(levels(blocks))],
layout, between, main,
pch=trellis.par.get()$superpose.symbol$pch)

panel.ancova(x, y, subscripts, groups,
transpose = FALSE, ...,
coef, contrasts, classes,
ignore.groups, blocks, blocks.pch, blocks.cex, pch)

## The following are ancova methods for generic functions.
## S3 method for class 'ancova':
anova(object, ...)

## S3 method for class 'ancova':
predict(object, ...)

## S3 method for class 'ancova':
print(x, ...) ## prints the anova(x) and the trellis attribute

## S3 method for class 'ancova':
model.frame(formula, ...)

## S3 method for class 'ancova':
summary(object, ...)

## S3 method for class 'ancova':
plot(x, y, ...) ## standard lm plot.  y is always ignored.

## S3 method for class 'ancova':
coef(object, ...)
##### Arguments
formula
A formula specifying the model.
data.in
A data frame in which the variables specified in the formula will be found. If missing, the variables are searched for in the standard way.
...
Arguments to be passed to aov, such as subset or na.action.
x
Covariate in ancova, needed for plotting when the formula does not include x. "aov" object in print.ancova, to match the argument of the print generic function. Variable to plo
groups
Factor. Needed for plotting when the formula does not include groups after the conditioning bar "|".
transpose
S-Plus: The axes in each panel of the plot are transposed. The analysis is identical, just the axes displaying it have been interchanged. R: no effect.
display.plot.command
The default setting is usually what the user wants. The alternate value TRUE prints on the console the command that draws the graph. This is strictly for debugging the ancova command.
superpose.level.name
Name used in strip label for superposed panel.
ignore.groups
When TRUE, an additional panel showing all groups together with a common regression line is displayed.
ignore.groups.name
Name used in strip label for ignore.groups panel.
pch
Plotting character for groups.
blocks
Additional factor used to label points in the panels.
blocks.pch
Alternate set of labels used when a blocks factor is specified.
blocks.cex
Alternate set of cex used when a blocks factor is specified.
layout
The layout of multiple panels. The default is a single row. See details.
between
Space between the panels for the individual group levels and the superpose panel including all groups.
main
Character with a main header title to be done on the top of each page.
y,subscripts
In "panel.ancova",#ifndef S-Plus see panel.xyplot. #endif #ifdef S-Plus see both xyplot and
object
An "aov"
coef, contrasts, classes
Internal variables used to communicate between ancova and panel.ancova. They keep track of the constant or different slopes and intercepts in each panel of the plot.
##### Details

The ancova function does two things. It passes its arguments directly to the aov function and returns the entire aov object. It also rearranges the data and formula in its argument and passes that to the xyplot function. The trellis attribute is a trellis object consisting of a series of plots of y ~ x. The left set of panels is conditioned on the levels of the factor groups. The right panel is a superpose of all the groups.

##### Value

• The result object is an ancova object which consists of an ordinary aov object with an additional trellis attribute. The default print method is to print both the anova of the object and the trellis attribute.

##### References

Heiberger, Richard M. and Holland, Burt (2004b). Statistical Analysis and Data Display: An Intermediate Course with Examples in S-Plus, R, and SAS. Springer Texts in Statistics. Springer. ISBN 0-387-40270-5.

ancova-class aov xyplot. See ancovaplot for a newer set of functions that keep the graph and the aov object separate.

##### Aliases
• ancova
• analysis of covariance
• covariance
• anova.ancova
• predict.ancova
• print.ancova
• model.frame.ancova
• summary.ancova
• plot.ancova
• coef.ancova
• panel.ancova
##### Examples
data(hotdog)

## y ~ x                     ## constant line across all groups
ancova(Sodium ~ Calories,     data=hotdog, groups=Type)

## y ~ a                     ## different horizontal line in each group
ancova(Sodium ~            Type, data=hotdog, x=Calories)

## This is the usual usage
## y ~ x + a  or  y ~ a + x  ## constant slope, different intercepts
ancova(Sodium ~ Calories + Type, data=hotdog)
ancova(Sodium ~ Type + Calories, data=hotdog)

## y ~ x * a  or  y ~ a * x  ## different slopes, and different intercepts
ancova(Sodium ~ Calories * Type, data=hotdog)
ancova(Sodium ~ Type * Calories, data=hotdog)

## y ~ a * x ## save the object and print the trellis graph
hotdog.ancova <- ancova(Sodium ~ Type * Calories, data=hotdog)
attr(hotdog.ancova, "trellis")

## label points in the panels by the value of the block factor
data(apple)
ancova(yield ~ treat + pre, data=apple, blocks=block)

## for a composite graph illustrating the four models listed above.