*! version 1.1.2 Mar2008
help dunnett

Title

dunnett -- The Dunnett method for a comparison
of all treatment means with a control mean

Syntax

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

Main
* control(#) set control value of factor_var

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

onesided set interval type to one-sided; the default is two-sided intervals

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 group means to a control group mean by
implementing Dunnett's test. 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 and requires that the degrees of freedom 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. Taking the conservative approach, the dunnett
command rounds the degrees of freedom to the lower value given in the tables.

The tables produced by this command may be wider than the number of default
columns set in Stata. You may want to increase the number of columns so that
the tables will properly display in the Results window.

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 numeric and a value of
factor_var.

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

onesided specifies that one-sided simultaneous confidence intervals be constructed.
The default is to construct two-sided simultaneous confidence intervals. When onesided is specified,
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 factor_var 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) onesided

One and two-sided 99% simultaneous confidence intervals

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

. dunnett response clinic, control(1) onesided level(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

References

Dunnett, C.W. (1955). “A multiple comparisons procedure
for comparing several treatments with a control.”
Journal of the American Statistical Association 509, 1096-1121.

Dunnett, C.W. (1964). “New tables for multiple comparisons
with a control.” Biometrics 20 482-491

Kuehl, R.O. (1994). Design of Experiments: Statistical
Principles of Research Design and Analysis. 2nd edition

Author

Theresa Boswell

tmb@stata.com

Also see

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