Is this what you were expecting?
> by(x, x$quiz, function(.sub) t.test(percent ~ group, data=.sub))
x$quiz: 1
Welch Two Sample t-test
data: percent by group
t = 6.3228, df = 6.231, p-value = 0.0006306
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
30.27235 67.94193
sample estimates:
mean in group High mean in group Low
92.85714 43.75000
------------------------------------------------------------
x$quiz: 2
Welch Two Sample t-test
data: percent by group
t = 1.591, df = 5.875, p-value = 0.1638
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-14.03856 65.46713
sample estimates:
mean in group High mean in group Low
65.71429 40.00000
------------------------------------------------------------
x$quiz: 3
Welch Two Sample t-test
data: percent by group
t = -0.1541, df = 3.506, p-value = 0.886
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-66.87207 60.20541
sample estimates:
mean in group High mean in group Low
71.66667 75.00000
>
On 10/22/07, Matthew Dubins <[EMAIL PROTECTED]> wrote:
> Hi,
>
> Following please find *some* of my data.
>
> percent quiz group
> 100 1 High
> 100 1 High
> 100 1 High
> 25 1 Low
> 50 1 Low
> 75 1 High
> 50 1 Low
> 75 1 High
> 100 1 High
> 100 1 High
>
> 50 1 Low
> 60 2 High
> 20 2 High
> 100 2 High
> 60 2 Low
> 40 2 Low
> 80 2 High
> 20 2 Low
> 60 2 High
> 40 2 High
> 100 2 High
>
> 90 3 High
> 100 3 High
> 0 3 High
> 90 3 Low
> 80 3 High
> 60 3 Low
> 100 3 High
> 60 3 High
>
>
> Here is the by command i used:
>
> by(percent, quiz, function(percent) {t.test(percent~group,
> data=marks.long)})
>
> Basically it gave me three of the same t.tests based on the full data
> set, not the subsets (quiz 1, 2 and 3).
>
> So far it seems like my method is more straightforward!
>
>
>
> Julian Burgos wrote:
> > Could you post some of your data and your initial test, and explain
> > why it didn't worked? It is difficult to figure out what is the
> > problem with your call to by().
> >
> > Julian
> >
> > Matthew Dubins wrote:
> >> I've tried to use by(), but the closest i got to it doing what I
> >> wanted was using the following:
> >>
> >> by(percent, quiz, function(percent) {t.test(percent~group,
> >> data=marks.long)})
> >>
> >> But the results it gave me weren't t.tests of percent by group
> >> according to quiz number.
> >>
> >>
> >> Julian Burgos wrote:
> >>> See by()
> >>>
> >>> Matthew Dubins wrote:
> >>>> Hi all,
> >>>>
> >>>> I wrote a simple function that gives me multiple t.test results
> >>>> according to a subset variable and am wondering whether or not I
> >>>> reinvented the wheel. Observe:
> >>>>
> >>>> t.test.sub <- function (formula, data, sub, ...)
> >>>> {
> >>>> for(i in 1:max(sub))
> >>>> {
> >>>> print(t.test(formula, data = subset(data, sub ==
> >>>> i), ...))
> >>>> }
> >>>> }
> >>>>
> >>>> Is there already a similar function in some package?
> >>>>
> >>>> Thanks,
> >>>> Matthew Dubins
> >>>>
> >>>> ______________________________________________
> >>>> R-help@r-project.org 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.
> >>>
> >>
> >
>
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org 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.
>
--
Jim Holtman
Cincinnati, OH
+1 513 646 9390
What is the problem you are trying to solve?
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