Re: t-test gets NaN instead of 0 for significance on x86-64

[John Darrington]

It looks good to me now. (I haven't tested it).

J'

On Fri, Sep 04, 2009 at 09:06:47AM -0700, Ben Pfaff wrote:
     John Darrington <address@hidden> writes:
     
     > On Thu, Sep 03, 2009 at 08:10:31PM -0700, Ben Pfaff wrote:
     >      John Darrington <address@hidden> writes:
     >      
     >      > As a short term solution, the best I can suggest is that we 
     >      > clamp pow2(correlation) to 1.0.
     >      
     >      Thanks.  I pushed out this stopgap fix.
     >
     > Isn't there still a problem when pairs[i].correlation is less than
     > -1 ?  For example -1.00000001
     
     You are right, of course.
     
     Could you check that this re-fix looks correct?
     
     commit 46912334775c83a23902bd5c7d72cd4ad8d23c95
     Author: Ben Pfaff <address@hidden>
     Date:   Fri Sep 4 09:05:08 2009 -0700
     
         T-TEST: Also fix the case where the correlation is slightly less than 
-1.0.
         
         Thanks to John Darrington for pointing out the omission.
     
     diff --git a/src/language/stats/t-test.q b/src/language/stats/t-test.q
     index dd40de9..ba7e938 100644
     *** a/src/language/stats/t-test.q
     --- b/src/language/stats/t-test.q
     ***************
     *** 1432,1447 ****
       
             double df = pairs[i].n -2;
       
     !       /* pairs[i].correlation is a correlation, so mathematically it will
     !          always be in the range [-1.0, 1.0].  Inaccurate calculations 
sometimes
     !          cause it to be slightly greater than 1.0, however, which makes 
the
     !          sqrt() below to come out as NaN instead of 0.  So force it to 
be 1.0
     !          or less. */
     !       double corr = MIN (1.0, pairs[i].correlation);
     ! 
             double correlation_t =
        pairs[i].correlation * sqrt (df) /
     !  sqrt (1 - pow2 (corr));
       
       
             /* row headings */
     --- 1432,1444 ----
       
             double df = pairs[i].n -2;
       
     !       /* corr2 will mathematically always be in the range [0, 1.0].  
Inaccurate
     !          calculations sometimes cause it to be slightly greater than 
1.0, so
     !          force it into the correct range to avoid NaN from sqrt(). */
     !       double corr2 = MIN (1.0, pow2 (pairs[i].correlation));
             double correlation_t =
        pairs[i].correlation * sqrt (df) /
     !  sqrt (1 - corr2);
       
       
             /* row headings */
     
     -- 
     Ben Pfaff 
     http://benpfaff.org
     
     
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