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Re: st: _tabi, exact_ gives obviously wrong P-value


From   Marcello Pagano <[email protected]>
To   [email protected]
Subject   Re: st: _tabi, exact_ gives obviously wrong P-value
Date   Wed, 22 Dec 2004 12:52:43 -0500

. tabi 27879 35177 29377 17201 \ 118 187 183 65, e chi2

          |                     col
      row |         1          2          3          4 |     Total
-----------+--------------------------------------------+----------
        1 |    27,879     35,177     29,377     17,201 |   109,634
        2 |       118        187        183         65 |       553
-----------+--------------------------------------------+----------
    Total |    27,997     35,364     29,560     17,266 |   110,187

         Pearson chi2(3) =  17.7073   Pr = 0.001
          Fisher's exact =                 0.001

Hmmmm  It worked for me?

m.p.


Tom Trikalinos wrote:

Dear all,
Perhaps I miss something, but

. tabi 27879 35177 29377 17201 \ 118 187 183 65, e chi2
yields Fisher's exact = 1.000
Pearson's Chi2(3) =17.7, Pr =0.001

StatXact calculates the P-value as 0.0005
The command seems to work properly for smaller numbers or different table geometries.


PS.
1. I have a fully updated Intercooled STATA 8.2 (ado & executable) running on a fully updated Mac (Panther)
2. There is no _tabi_ command in my personal directory overriding the updated _tabi_
. which tabi
/Applications/Stata/ado/updates/t/tabi.ado
*! version 2.0.5 13jul2004
3. the same table as 4x2 gives the same numbers
4. the command exits with _rc 0 --> hence all's OK, right?
5. Probably there's no excuse for Fisher's exact in this table, but let's not stay there

Tom Trikalinos, MD
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