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Date:         Fri, 25 Feb 2000 12:04:33 -0600
Reply-To:     Thompson Bill T Contr USAFSAM/FEC <Bill.Thompson@BROOKS.AF.MIL>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Thompson Bill T Contr USAFSAM/FEC <Bill.Thompson@BROOKS.AF.MIL>
Subject:      Re: post hoc tests for unequal N
Comments: To: "Nichols, David" <nichols@SPSS.COM>
Content-Type: text/plain; charset="iso-8859-1"

David, on your first reply regarding the help message, I stand corrected in my use of the term designed. Regarding your second message I am really confused. If not Waller-Duncan or Tukey b what post hoc test would you recommend for unequal sample sizes? I don't feel the need for both tests to afford exactly the same results, however, when they don't one must decide which results to use, that is the answer I am seeking. Of course if you have received the other message I sent, after this one, you would understand my concern about using the right post hoc test.

Thanks, Bill

-----Original Message----- From: Nichols, David [mailto:nichols@SPSS.COM] Sent: Friday, February 25, 2000 11:59 AM To: SPSSX-L@LISTSERV.UGA.EDU Subject: Re: post hoc tests for unequal N

I don't see why you'd expect these to always give similar results. They're different tests using different criteria. I wouldn't use either one with unequal N.

David Nichols Principal Support Statistician and Manager of Statistical Support SPSS Inc.

> -----Original Message----- > From: Thompson Bill T Contr USAFSAM/FEC > [mailto:Bill.Thompson@BROOKS.AF.MIL] > Sent: Friday, February 25, 2000 11:35 AM > To: SPSSX-L@LISTSERV.UGA.EDU > Subject: Re: [SPSSX-L] post hoc tests for unequal N > > > David, I just ran the MANOVA again and asked for both the Tukey b and > Waller-Duncan post hoc tests. Although for many of the variables the > results are equivalent for many of the tests the results are quite > different. Again forgive my ingorance and can someone > explain to me why the > results are so different and which test results are the most > appropriate to > report. I realize this may seem somewhat trivial to some, > but in my world > differences can me the difference between life and death, > teating or not > treating. Therefore, again, please forgive my ignorance and > any help is > greatly appreciated. > > Thanks in advance, > > Bill T. > > -----Original Message----- > From: Nichols, David [mailto:nichols@SPSS.COM] > Sent: Thursday, February 24, 2000 5:09 PM > To: SPSSX-L@LISTSERV.UGA.EDU > Subject: Re: post hoc tests for unequal N > > > I don't know exactly what Statistica has implemented, but > there's no reason > to expect Spjotvoll-Stoline results to match those from > REGWF. The closest > thing that SPSS has to a Spjotvoll-Stoline approach would be > a Tukey-Kramer, > which is what you get for the pairwise comparisons output > with unequal Ns > when you specify Tukey. Since this is always as powerful as the > Spjotvoll-Stoline and maintains Type I error protection, I > see no reason to > use a Spjotvoll-Stoline as opposed to a Tukey-Kramer. > > All of the methods for homogeneous subsets in SPSS use the > harmonic mean of > the sample sizes in order to maintain monotonicity of > critical values and > logical rationale of the stepdown tests (these methods were > designed for > equal sample sizes). They generally do not maintain Type I > error protection > with unequal N and are therefore not recommended for that > case (we actually > print a footnote indicating that Type I error levels are not > guaranteed). > > There are further problems with the REGWF approach. Even for > equal sample > sizes, the critical values are not always monotonic, leading > to situations > where applying the test correctly is difficult or impossible. > Jason Hsu > discusses this in his Multiple Comparisons: Theory and > Methods. He brought > this to my attention some time ago and we did some tweaking > in response, but > it doesn't really solve the problem, so I'd be very hesitant > to recommend > that method. Interestingly enough, I just tried his example > in Release 8 of > SAS, and requesting the REGWF method on the MEANS statement > in PROC GLM > produces the following: "NOTE: The REGWF option has been > disabled and will > no longer be available. You can use the REGWQ option instead." > > David Nichols > Principal Support Statistician and > Manager of Statistical Support > SPSS Inc. > > > -----Original Message----- > > From: Thompson Bill T Contr USAFSAM/FEC > > [mailto:Bill.Thompson@BROOKS.AF.MIL] > > Sent: Wednesday, February 23, 2000 11:25 AM > > To: SPSSX-L@LISTSERV.UGA.EDU > > Subject: [SPSSX-L] post hoc tests for unequal N > > > > > > I ran a MANOVA with 13 DV's and 1 IV (Group with 5 levels). > > The overall > > MANOVA as significant at .00001. The five levels of Group have the > > following N's: > > > > Group N > > 1 148 > > 2 5051 > > 3 134 > > 4 69 > > 5 103 > > > > Can someone tell me which Post Hoc test would be the most > > appropriate to use > > in SPSS. Statistica provides the Tukey HSD for unequal N's > > (Spjotvoll/Stoline test) but the results are significantly > (excuse the > > choice of word) different from those of the REGW F performed > > in SPSS. My > > ignorance causes me some concern and I would appreciate any > > insight and > > recommendations on performing Post Hoc tests with unequal N' > > and possibly > > making sense of the REGW F output in SPSS. > > > > Thanks in advance for your assistance, > > > > Bill Thompson - Biostatistician > > Conceptual MindWorks Inc. > > School of Aerospace Medicine > > Aeromedical Consult Service - USAFSAM/FEC > > 2507 Kennedy Circle > > Brooks Air Force Base, Texas 78247 - 5117 > > DSN: 240-6835 Comm: 536-6835 > > Fax: DSN/Comm - 2817 > > email: bill.thompson@brooks.af.mil > > >


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