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st: RE: Fisher's exact P and -csi-


From   "FEIVESON, ALAN H. (AL) (JSC-SK) (NASA)" <alan.h.feiveson@nasa.gov>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Fisher's exact P and -csi-
Date   Wed, 17 Mar 2004 08:20:36 -0600

Garry - Unless the use of commands -tabi- and -csi- have changed, it appears
that you discovered something wrong in Stata 8. In Stata 7, one gets correct
results, both for -tabi- and -csi-:

Al Feiveson
==================================================================
[Stata 7]
. tabi 3 0 \ 0 3

           |          col
       row |         1          2 |     Total
-----------+----------------------+----------
         1 |         3          0 |         3 
         2 |         0          3 |         3 
-----------+----------------------+----------
     Total |         3          3 |         6 

           Fisher's exact =                 0.100
   1-sided Fisher's exact =                 0.050

. csi 3 0  0 3,e

                 |   Exposed   Unexposed  |     Total
-----------------+------------------------+----------
           Cases |         3           0  |         3
        Noncases |         0           3  |         3
-----------------+------------------------+----------
           Total |         3           3  |         6
                 |                        |
            Risk |         1           0  |        .5
                 |                        |
                 |      Point estimate    |  [95% Conf. Interval]
                 |------------------------+----------------------
 Risk difference |                1       |         1           1  
      Risk ratio |                .       |         .           .  
 Attr. frac. ex. |                1       |         .           .  
 Attr. frac. pop |                1       |
                 +-----------------------------------------------
                                1-sided Fisher's exact P = 0.0500
                                2-sided Fisher's exact P = 0.1000

=====================================================================. 
Stata 8:
. csi 3 0 0 3,e

                 |   Exposed   Unexposed  |     Total
-----------------+------------------------+----------
           Cases |         3           0  |         3
        Noncases |         0           3  |         3
-----------------+------------------------+----------
           Total |         3           3  |         6
                 |                        |
            Risk |         1           0  |        .5
                 |                        |
                 |      Point estimate    |  [95% Conf. Interval]
                 |------------------------+----------------------
 Risk difference |                1       |         1           1  
      Risk ratio |                .       |         .           .  
 Attr. frac. ex. |                1       |         .           .  
 Attr. frac. pop |                1       |
                 +-----------------------------------------------
                                1-sided Fisher's exact P = 0.0000
                                2-sided Fisher's exact P = 0.0000

. tabi 3 0 \ 0 3

           |          col
       row |         1          2 |     Total
-----------+----------------------+----------
         1 |         3          0 |         3 
         2 |         0          3 |         3 
-----------+----------------------+----------
     Total |         3          3 |         6 

           Fisher's exact =                 0.000
   1-sided Fisher's exact =                 0.000







-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Garry Anderson
Sent: Wednesday, March 17, 2004 4:38 AM
To: statalist@hsphsun2.harvard.edu
Subject: st: Fisher's exact P and -csi-


Hi,

I am enquiring why the value of Fisher's exact P for a 2 x 2 table should 
decrease when there is a smaller sample size for each of the two samples 
and the two proportions remain at 100% and 0%?

For example
-csi 6 0 0 6,e-
                                1-sided Fisher's exact P = 0.0011
                                2-sided Fisher's exact P = 0.0022

-csi 5 0 0 5,e-
                                 1-sided Fisher's exact P = 0.0000
                                 2-sided Fisher's exact P = 0.0000

-csi 4 0 0 4,e-
                                 1-sided Fisher's exact P = 0.0143
                                 2-sided Fisher's exact P = 0.0286

-csi 3 0 0 3,e-
                                 1-sided Fisher's exact P = 0.0000
                                 2-sided Fisher's exact P = 0.0000

-csi 2 0 0 2,e-
                                 1-sided Fisher's exact P = 0.0000
                                 2-sided Fisher's exact P = 0.0000

-csi 1 0 0 1,e-
                                 1-sided Fisher's exact P = 0.5000
                                 2-sided Fisher's exact P = 1.0000

Any suggestions would be appreciated.

(SPSS gives P=0.1 for the 3 0 0 3 combination)

I am using Stata 8.2, 30 Jan 2004, ado 11 Mar 2004.

Kind regards, Garry

Garry Anderson
School of Veterinary Science
University of Melbourne
250 Princes Highway           Ph  03 9731 2221
WERRIBEE    3030             Fax  03 9731 2388
Email:  g.anderson@unimelb.edu.au 

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