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
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/