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 [EMAIL PROTECTED] (801) 408-8111 > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Daniel Stahl > Sent: Tuesday, August 14, 2007 7:11 AM > To: r-help@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 > > > > > > > > -- > > ______________________________________________ > R-help@stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
