# [R] Odp: Problem with "by": does not work with ttest (but with lme)

Petr PIKAL petr.pikal at precheza.cz
Tue Aug 14 15:35:13 CEST 2007

```Hi

r-help-bounces at stat.math.ethz.ch napsal dne 14.08.2007 15:11:11:

> 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))

by(warpbreaks,tension,function(x)t.test(breaks ~ wool,data=x,paired =
TRUE))

Regards
Petr

> *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 at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help