Date: Wed, 14 Mar 2007 17:30:47 -0800
Reply-To: David L Cassell <davidlcassell@MSN.COM>
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: David L Cassell <davidlcassell@MSN.COM>
Subject: Re: bimodal
Content-Type: text/plain; format=flowed
> >Does anyone know if there is a way to detect bi or multi modal
> >distributions using surveyselect, or univariate, or mean?
>There is a dip test invented by Hartigan. (google those three words, get
>lots of refs), and it seems to be available in SAS (add SAS to the google
>search above.....but I am away from the office today, and don't have time
>to check which of what Google supplies is good.....
Hartigan's dip test really tests unimodal against 'other', although it is
used against a bimodal alternative. It won't test bimodal against all
(unimodal, trimodal, ...).
There's an R package that will do Hartigan's dip test for you. The R
has a much more robust set of p-values, based on a much larger simulation
set than Hartigan & Hartigan used back in 1985.
And there's also a test by Cheng and Hall (1999, Annals of Statistics).
But the bottom line is: if you can see it, you can find it with the test; if
can't be sure with your eyes, you can't be sure with the test either.
a rule that holds for an awful lot of stat tests, not just this one.
David L. Cassell
3115 NW Norwood Pl.
Corvallis OR 97330
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