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From |
Weihua Guan <wguan@stata.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: Re: Cornfield and Woolf confused? -cc- vs -exactcc- vs Breslow&Day |

Date |
Mon, 02 Dec 2002 14:11:10 -0600 |

--Michel Camus <Michel_Camus@hc-sc.gc.ca> > Stata's -cc- and Dupont & Plummer's -exactcc- do not provide the same > Cornfield confidence intervals for ORs. > Moreover, Stata's -cci- does not provide the same results as those by > Breslow & Day (Stat.Methods in Cancer Research-Vol.1, p.124, 135-6) > neither. Why is it so? Is their a mistake in Stata's -cci- ? > [...] First, let's clarify what -cci- produces. In Stata 7, -cci- calculates the exact confidence intervals by default, and calculates the unadjusted Cornfield confidence intervals when -cornfield- is specified. Now we can compare the results from -cci- and -exactcci- using Michel's example: 1) exact confidence intervals: . cci 96 104 109 666 [...] Odds ratio | 5.640085 | 3.937435 8.061794 (exact) [...] . exactcci 96 104 109 666, exact [...] | | Exact limits | | 3.937467 8.061784 [...] These two commands actually apply the same algorithm in computing the upper and lower limits. The slight difference is due to computational precision. If both program stored the intermediate results as double precision or scalars, they would return identical results. 2) Cornfield confidence intervals: . cci 96 104 109 666, cornfield [...] Odds ratio | 5.640085 | 4.003217 7.94673 (Cornfield) [...] . exactcci 96 104 109 666, exact [...] | | Cornfield's limits Odds ratio | 5.640085 | 3.942972 8.071281 Adjusted | | 3.937435 8.061794 Unadjusted [...] The diffenence is due to the change of -cci-'s behavior. In Stata 6, -cci- computes (unadjusted) Cornfield confidence intervals by default, but switch the default to the exact CI in Stata 7. Since -exactcci- calls -cci- to grab the unadjusted CI, it in fact gets the exact CI while expecting Cornfield. If we run this command in Stata 6, the correct values will be returned: . exactcci 96 104 109 666, exact [...] | | Cornfield's limits Odds ratio | 5.640085 | 3.942972 8.071281 Adjusted | | 4.003217 7.94673 Unadjusted [...] The adjusted Cornfield CI should be calculated correctly in -exactcci-. The difference between "adjusted" and "unadjusted" Cornfield CIs is a term +/-0.5 in the iterative process. The formula for adjusted CI is given in Schlesselman's "Case-Control Studies" (1982) p.177 (7.10). Weihua Guan <wguan@stata.com> Stata Corp. * * 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/

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