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Date:   Thu, 1 Oct 2009 16:42:27 +0200
Reply-To:   García-Granero <mgarciagranero@gmail.com>
Sender:   "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:   García-Granero <mgarciagranero@gmail.com>
Subject:   Runs test: cut point at mean or median
Content-Type:   text/plain; charset=ISO-8859-1; format=flowed

Hi everybody

I am using a very simple dataset (serum Fe levels with an automatic analyzer) to explain the concept of randomness and runs test:

DATA LIST FREE /Fe (F8.2). BEGIN DATA 2.6 2.4 2.7 3.2 3.4 3.5 3.2 3.6 3.4 3.5 END DATA.

If I run the test using as cut-off the median, the test is non significant. If I use the mean, the result is significant.

NPAR TESTS /RUNS(MEDIAN)=Fe /RUNS(MEAN)=Fe /METHOD=EXACT TIMER(5).

Moreover, using Hart's (*) modification of the mean square successive difference test (assumes normally distributed variables), the result is also significant. Since I know the question my students will ask is: which cut-point is better, mean or median? I'm trying to get an answer before the class. Suggestions and comments are welcome and very appreciated..

(*) Ref: http://books.google.es/books?id=kZ5NoA2BwjEC&pg=PA212&dq=test+de+hart+aleatoriedad+luna+del+castillo#v=onepage&q=&f=false

* Macro for Hart's randomness test *. DEFINE HART(!POS !TOKENS(1)). MATRIX. PRINT/TITLE='HART (mean square successive difference) TEST'. GET X /VAR=!1 /NAME=vname /MISSING=OMIT. COMPUTE N=NROW(x). COMPUTE Diff2=MAKE(N-1,1,0). LOOP i=1 TO N-1. . COMPUTE Diff2(i)=(x(i)-x(i+1))**2. END LOOP. COMPUTE SC=CSSQ(x)-(CSUM(x))**2/N. COMPUTE H=CSUM(Diff2)/SC. COMPUTE Sigma=2*SQRT((n-2)/(n**2-1)). COMPUTE ZValue=(H-2)/Sigma. COMPUTE PValue=1-CDFNORM(ABS(ZValue)). PRINT {H,ZValue,PValue,2*PValue} /FORMAT='F8.3' /CLABEL='H','Z','1-tail P','2-tail P' /RNAME=vname /TITLE='Statistics'. END MATRIX. !ENDDEFINE.

HART Fe.

Best regards, Marta GG

-- For miscellaneous SPSS related statistical stuff, visit: http://gjyp.nl/marta/

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