[R] Problem with "by": does not work with ttest (but with lme)
Daniel Stahl
daniel_stahl at operamail.com
Tue Aug 14 15:11:11 CEST 2007
Hello,
I would like to do a large number of e.g. 1000 paired ttest using the by-function. But instead of using only the data within the 1000 groups, R caclulates 1000 times the ttest for the full data set(The same happens with Wilcoxon test). However, the by-function works fine with the lme function.
Did I just miss something or is it really not working? If not, is there any other possibility to avoid loops?
Thanks
Daniel
Here is the R help example for "by"
require(stats)
attach(warpbreaks)
by(warpbreaks, tension, function(x) lm(breaks ~ wool, data = x))
*->works great
by(warpbreaks,tension,function(x)t.test(breaks ~ wool,data=warpbreaks,paired = TRUE))
*Same output for each level of tension:
tension: L
Paired t-test
data: breaks by wool
t = 1.9956, df = 26, p-value = 0.05656
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.1735803 11.7291358
sample estimates:
mean of the differences
5.777778
------------------------------------------------------------------------
tension: M
Paired t-test
data: breaks by wool
t = 1.9956, df = 26, p-value = 0.05656
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.1735803 11.7291358
sample estimates:
mean of the differences
5.777778
------------------------------------------------------------------------
tension: H
Paired t-test
data: breaks by wool
t = 1.9956, df = 26, p-value = 0.05656
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.1735803 11.7291358
sample estimates:
mean of the differences
5.777778
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