[R-lang] Dunnett test
lngmyers at ccu.edu.tw
Wed May 16 19:51:26 PDT 2007
Michael T Hammond wrote:
> Does anybody know if there is a way to get the "Dunnett test" (Dunnett,
> 1955) in R?
> I've found references to it in the "multcomp" package, but not the test
> per se.
My first response was -- with proper design, you don't have to worry
about post hoc tests! But actually, comparing multiple things with a
single control is pretty common in priming experiments....
Anyway, I think I may have figured it out, but I'll let the real experts
check to be sure.
Look at the demo "recovery" in the "multcomp" manual. Modify the code
like so, to make the test bidirectional with alpha = 0.05:
amod <- aov(minutes ~ blanket, data = recovery)
ht <- glht(amod, linfct = mcp(blanket = "Dunnett"))
confint(ht, level = 0.95)
This gives you the following 95% confidence intervals:
Estimate lwr upr
b1 - b0 == 0 -2.1333 -6.1245 1.8578
b2 - b0 == 0 -7.4667 -11.4578 -3.4755
b3 - b0 == 0 -1.6667 -3.8685 0.5352
Notice that the function is treating b0 as the "control", since it's
alphabetically the first level. The only comparison that has a
confidence interval NOT crossing 0 (i.e. both signs are the same) is the
one for b2, so that's the only significant comparison.
I compared this result with what's given by StatView for the same data,
but with "b0" renamed as "zb0", since for some reason StatView treats
the alphabetically LAST level as the control. Then I get:
Dunnett for minutes
Significance Level: 5 %
Mean Diff. Crit. Diff.
b1 -2.133 3.928
b2 -7.467 3.928 S
b3 -1.667 2.167
Note that only b2 is marked "S" (significant).
Did I do this right...?
Graduate Institute of Linguistics
National Chung Cheng University
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Email: Lngmyers at ccu.edu.tw
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