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 (email@example.com) wrote:
: 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
Rich Ulrich, biostatistician firstname.lastname@example.org
http://www.pitt.edu/~wpilib/index.html Univ. of Pittsburgh