| Re: t-testing in IDL [message #58303 is a reply to message #58240] |
Thu, 24 January 2008 06:52  |
mmiller3
Messages: 81
Registered: January 2002
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Member |
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>>>> > "mariamalene" == mariamalene <mariamalene@hotmail.com> writes:
> I have two sample data sets with 20 years of 12 months
> simulations in a lon/lat grid of 128 x 64. ,
> i.e. 2x(20,12,128,64). I would like to find the difference
> between the two simulations and find out which differences
> that are significant at a 95% level. I have calculated the
> annual average of my 20-year simulation, and also the
> standard- deviation, but how do I perfom the t-test? I have
> looked at the tm_test but haven't figured out how to apply
> it on my 4-D approach..
I'm not entirely clear on what you want to do, but I'll take a
shot.
If you have two data sets, A and B, and you want to test the
hypothesis that the means are the same, you can use tm_test as in
tm_test(A,B). It the data sets are paired, meaning A[i] is
correlated with B[i], you can use tm_test(A,B,/paired) to test
the hypothesis that the mean difference is zero. I suspect that
this might be what you want. If you want to test the hypothesis
that the mean difference is zero and that the difference does not
have a spatial dependence, you could use a mixed effect model.
If you are going to do stats, I'd suggest you use a statistics
package, such as R (www.r-project.org) instead of IDL. For
example, if a t-test is to be useful, your data must be normally
distributed. That is very easy to test with R, and relatively
easy to test with IDL, but once you get beyond very basic
statistics, you'll get beyond IDL's build-in capabilities. There
are relatively easy to use mixed effects modeling tools in R.
Mike
P.S. For the record: this posting should not be taken as an
opinion against IDL, which I love/hate and use constantly. It's
just that R, which I love/hate and use constantly, has a
different set of strenghts and weaknesses. Give me grep, awk,
python, IDL and R and an emacs to run it all in and I can shoot
myself in the other foot every time!
P.P.S. Getting even more off topic: this list needs to be
updated: http://burks.bton.ac.uk/burks/language/shoot.htm
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