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st: CC exact 95% confidence interval brackets 1 while the P value is < .05


From   Bill Warburton <bill.warburton@ubc.ca>
To   statalist@hsphsun2.harvard.edu
Subject   st: CC exact 95% confidence interval brackets 1 while the P value is < .05
Date   Wed, 27 Sep 2006 13:15:22 -0700

Can you tell me why the exact 95% confidence interval brackets 1 while the P
value is < .05?

In a logistic regression I was suspicious of the rather narrow confidence
intervals in the analysis of a relatively rare event so I checked using a
2x2 table with an exact test (results below) and the exact confidence
interval does indeed bracket 1, but the p-value given is less than .05(?).

Can you tell me how to interpret/report this?




. cc xp1 disease ,e;
                                                         Proportion
                 |   Exposed   Unexposed  |      Total     Exposed
-----------------+------------------------+------------------------
           Cases |         2          99  |        101       0.0198
        Controls |         7        3315  |       3322       0.0021
-----------------+------------------------+------------------------
           Total |         9        3414  |       3423       0.0026
                 |                        |
                 |      Point estimate    |    [95% Conf. Interval]
                 |------------------------+------------------------
      Odds ratio |           9.5671       |    .9562236    50.98143 (exact)
 Attr. frac. ex. |         .8954751       |   -.0457805     .980385 (exact)
 Attr. frac. pop |         .0177322       |
                 +-------------------------------------------------
                                  1-sided Fisher's exact P = 0.0271
                                  2-sided Fisher's exact P = 0.0271

Other information: Bitest assuming that the true proportion is .0021 gives P
= 0.019410 which is very close to the probability of observing 2 or more
cases out of 101 exposed in a Poisson distribution with mean .0021.

Bill 

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