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Date:         Wed, 13 Aug 1997 19:37:33 GMT
Reply-To:     Richard F Ulrich <wpilib+@PITT.EDU>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@UGA.CC.UGA.EDU>
From:         Richard F Ulrich <wpilib+@PITT.EDU>
Organization: University of Pittsburgh
Subject:      Re: significance of correlation coefficient

Richard Batt (rbatt@arts.adelaide.edu.au) wrote: : Hi!

: For a simple correlation (N=35) SPSS 7 for Windows gives p = 0.05 for : approx r = 0.330. However when I check by hand using, : p_val = 1 - CDF.T(t,33) where t = r * sqrt( (33/(1-r^2) ) : I get p = 0.05 for approx 0.295. Any suggestions as to why the : discrepancy? In my simulations I was doing multiple correlations, : perhaps SPSS adjusts the p value for this?

- If you are doing MULTIPLE CORRELATIONs then you are looking at multiple Rs which are inherently two-tailed. You have described your first correlation in contradictory ways, when you say "simple" and then "multiple".

The CDF gives you one tail, which yields a one-tailed p-value, as you show above.

Rich Ulrich, biostatistician wpilib+@pitt.edu http://www.pitt.edu/~wpilib/index.html Univ. of Pittsburgh


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