# [R] dunnett test questions

Achim Zeileis Achim.Zeileis at uibk.ac.at
Mon Oct 28 07:40:51 CET 2013

```On Sun, 27 Oct 2013, Imran Akbar wrote:

> Hi,
>    I've got a data set with a control group and a number of experimental
> groups, that have unequal sample sizes, and am measuring the number of
> people in each that respond yes or no.  I'd like to use a dunnett test in
> R, where the syntax is supposed to be like:
>
> library(multcomp)
> test.dunnett=glht(anova_results,linfct=mcp(method="Dunnett"))
> confint(test.dunnett)
> plot(test.dunnett)
>
> but:
> 1)  How do I run a dunnett test without doing the ANOVA (which wouldn't
> have its requirements satisfied, as the measurements are not independent
> due to the control group)?

But the control group is a separate independent group from the three
treatments A-C, isn't it? Then independence should not be a problem.

For the binary response you need something different than an ANOVA, e.g.,
an analysis of deviance in a logistic regression.

> 2)  Do I have to tell the test what my sample sizes are, or will it
> calculate the sums itself?

If you supply a suitable fitted model, then glht() can infer the group
sizes from that:

## data and table
dat <- data.frame(
freq = c(23, 19, 27, 53, 623, 523, 823, 469),
resp = factor(rep(c("Yes", "No"), each = 4)),
method = factor(rep(1:4, 2), labels = c("Control", "A", "B", "C"))
)
tab <- xtabs(freq ~ method + resp, data = dat)

## visualization and Pearson chi-squared test
spineplot(tab[, 2:1])
mosaicplot(tab, shade = TRUE, off = c(5, 0.5))
chisq.test(tab)

## Logistic regression and analysis of deviance chi-squared test
m <- glm(resp ~ method, weights = freq, data = dat, family = binomial)
anova(m, test = "Chisq")

## Odds ratios
exp(coef(m)[-1])

## Dunnett test
library("multcomp")
m_glht <- glht(m, linfct = mcp(method = "Dunnett"))
summary(m_glht)
confint(m_glht)
plot(m_glht)

> Here's my matrix:
>
>
>       Control        A           B           C
> Yes       23          19          27          53
> No        623         523         823         469
>
>
> thanks,
>
> imran
>
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>
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