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From |
Antoine Terracol <terracol@univ-paris1.fr> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: CI for a difference in proportions |

Date |
Wed, 27 Feb 2008 20:18:32 +0100 |

Dear _all, This might be a silly question, but I currently have no access to Stata's user manuals or stats textbook. I have data on two independant samples (of different sizes), each observed over 20 periods. I wish to draw a graph representing, for each period, the difference in the proportion of some event between the two groups, together with the 95% CI for this difference. I first wanted to use -prtest- for each period, store/compute the differences and CI bonds in new variables, and graph them against periods. However, I am a bit confused over which standard error to use (and how to get them) when computing the CI bounds. -prtest- gives two standard errors, the one under H0 can be easily computed from stored results r(P_1), r(P_2) and r(z). The standard errors for each proportion (se1 and se2) are not stored, and I do not know if the fact that my calculation of sqrt(se1^2 + se2^2) differs from the reported standard error for the difference (not under H0) is caused by rounding errors or if my formula is incorrect... I thought of using -proportion- to get the standard errors as stored results, but it actually gives standard errors for each proportion that differ from -prtest-, but are identical to the ones computed by -mean-. Any advice would be appreciated. Antoine * * 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/

**Follow-Ups**:**st: RE: CI for a difference in proportions***From:*"Newson, Roger B" <r.newson@imperial.ac.uk>

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