Ingrid Tohver wrote:> I would like to run a t-test within a "by" group function. My
> dataset, "error", is organized as the following (I have 133
Sites):
>
> Site week Dataset Region lat_map long_map mean_tsim diff20 diff40
> diff80
> ALFI 15 USACE UC 48.15625 -117.0938 8.87 1.34 1.90
> 2.98
> ALFI 16 USACE UC 48.15625 -117.0938 10.28 0.57 1.08
> 2.27
> ALFI 17 USACE UC 48.15625 -117.0938 11.08 0.74 1.30
> 2.52
> ALFI 18 USACE UC 48.15625 -117.0938 12.23 0.42 1.11
> 2.42
> ALFI 19 USACE UC 48.15625 -117.0938 13.19 1.00 1.73
> 3.14
> ALFI 20 USACE UC 48.15625 -117.0938 14.31 1.77 2.62
> 3.78
>
> I am interested in running the t-test by the "Site" index. My
code
> looks like this:
>
> t_test<-by(error, error['Site'], function(dat)
t.test(subset(error
> $diff20),subset(error$diff80), data=dat))
>
> This code runs the t-test, but over the whole dataset without
> discriminating by Site, so each Site's result is the same. Could
> someone help determine a better approach or why mine is not working.
I guess you want
by(error, error['Site'], function(dat) t.test(dat$diff20, dat$diff40))
Uwe Ligges
> Thank you,
> Ingrid
>
>
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>
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