*! version 1.1 Nov2007
help dunnett

Title

dunnett -- The Dunnett Method for a Comparison
of All Treatments with a Control

Syntax

dunnett
response_var
factor_var
[if] [in],
control(#)
[options]

Main
* control(#) set control value of factor_var

ci(#) set confidence level; the default is ci(95)

side(#) set interval type; the default is side(2)

nolabel show numeric codes, not labels

separator(#) draw separator line after every # groups; default
is separator(5)

missing treat missing values of factor_var as a category

* control(#) is required.

Description

The dunnett command compares all treatments to a control by implementing
the Dunnett Method. Simultaneous confidence intervals for differences between
each individual group mean and the control mean are reported using the tables
from Dunnett(1955) and Dunnett(1964).

Because the table values are used, the dunnett command can only construct 95% and 99%
simultaneous confidence intervals for and requires that the degree of freedom to be
at least five. The Dunnett Method also requires that all treatments have the same sample size
for the validity of the test to hold.

If a two-sided test is specified, the number of groups in factor_var must be less
than 21 including the control. If a one-sided test is specified, the number of groups
must be less than 10 including the control.

Options

Main

control specifies the control value of factor_var that dunnett will use
as the control to compare all other groups to. This value must be numberic and a value of
factor_var.

ci(#) specifies the confidence level, as a percentage, for simultaneous confidence
intervals. The default is ci(95) and can currently only be changed to ci(99).

side(#) specifies the type of test to be performed. The default is side(2) where
the two sided simultaneous confidence intervals are reported. When changed to side(1), both the
lower and upper simultaneous bounds are reported for a lower and upper tailed test respectively.

nolabel causes the numeric codes to be displayed rather than the value
labels in the reported table. The default is to show the value labels if they exist.

separator(#) specifies how often to insert separation lines into the output.
The default is separator(5), meaning that a line is drawn after every five treatments.
separator(10) would draw a line after every 10 treatments and separator(0) suppresses
the separation line.

missing requests that missing values of be
treated as a category rather than as observations to be omitted from the
analysis.

Examples

Setup

. webuse rash

Two-sided simultaneous confidence intervals

. dunnett response clinic, control(1)

One-sided simultaneous confidence intervals

. dunnett response clinic, control(1) side(1)

One and two-sided 99% simultaneous confidence intervals

. dunnett response clinic, control(1) ci(99)

. dunnett response clinic, control(1) side(1) ci(99)

Setup

. webuse rash

. replace clinic = . if clinic==8

Treat missing values as a category

. dunnett response clinic, control(1) missing

Saved results

dunnett saves the following in r():

Scalars

r(N) total number of observations

r(groups) number of groups including the control

r(df) degrees of freedom

r(mse) mean squared error

r(side) type of test

r(confidence) confidence level of SCI

r(reps) number of replications for each group

r(d) table value

r(criterion) Dunnett criterion

r(control) control value

Macros

r(fact_var) factor variable

r(resp_var) response variable

Author

Theresa Boswell

tmb@stata.com

Also see

Online: [R] oneway,
[R] anova,
[R] loneway