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.