Date: Wed, 13 Aug 1997 19:37:33 GMT
ReplyTo: Richard F Ulrich <wpilib+@PITT.EDU>
Sender: "SPSSX(r) Discussion" <SPSSXL@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/(1r^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 twotailed. 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 onetailed pvalue, as you
show above.
Rich Ulrich, biostatistician wpilib+@pitt.edu
http://www.pitt.edu/~wpilib/index.html Univ. of Pittsburgh
