[R] Problem with "by": does not work with ttest (but with lme)
Greg Snow
Greg.Snow at intermountainmail.org
Tue Aug 14 17:42:26 CEST 2007
I think at least part of your problem is that in the lm example you use
data=x (correct), but in the t.test example you use data=warpbreaks, so
in that case it is uing the full data set, not just the portion passed
by the 'by' function. Try the t.test example again with data=x and see
what happens.
Hope this helps,
--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at intermountainmail.org
(801) 408-8111
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Daniel Stahl
> Sent: Tuesday, August 14, 2007 7:11 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Problem with "by": does not work with ttest (but
> with lme)
>
> 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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