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From |
SeyiLSHTM <seyi.soremekun@lshtm.ac.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: rate ratios from xtlogit using margins - how? |

Date |
Fri, 17 Dec 2010 05:30:19 -0800 (PST) |

Dear Listers, I have what I hope is a simple question. I am running an xtlogit model (re) to compare stillbirth rates in an intervention (=1) and a control group (=0) with clustering. Stata outputs the odds ratio, though I'd rather convert this to show the adjusted rates, and an adjusted rate ratio or a rate difference (with CIs), as this is more intuitive when presenting results to mixed audiences. my model: xtlogit still intvn, re i(zone) or I used the margins command to obtain what I understood to be the predicted probabilities (adjusted rates) of stillbirth in the two groups of intvn (the binary var described above): . margins intvn intvn 0 .6546526 .0915509 7.15 0.000 .4752162 .8340891 1 1.028017 .0932528 11.02 0.000 .8452453 1.210789 I see xb (the linear prediction), not the probabilities - is this correct? In the stata manual it says for margins after logit: "....numbers reported in the “Margin” column are average predicted probabilities...", but I am assuming that this is not the same for xtlogit as the above numbers are clearly not probabilities, so I exponentiate the values; e(0.65) and e(1.03). This gives me the odds for each group, right? not the probabilities (as often greater than 1). So, after transforming xb, I think I now have the odds of stillbirth in intvn=0 and intvn=1, as the ratio of these matches exactly the odds ratio in my xtlogit model. So, to get adjusted rates (probabilities) and a rate ratio, I then transformed these odds to probabilities (0.654/(1+0.654), and 1.028/(1+1.028)) and found the ratio of these, 0.736/0.658 = 1.12 = rate ratio Is this correct? How can I obtain a single CI for this rate ratio? Can I use this formula to transform the upper and lower limits of the odds ratio: odds ratio CI/ (1- [CI in intvn=0 group]) + ([CI in intvn=0 group] * odds ratio CI) (Obtained using help/findit oddsrisk) Apologies for length, any advice appreciated. Have a good Christmas, Seyi -- View this message in context: http://statalist.1588530.n2.nabble.com/rate-ratios-from-xtlogit-using-margins-how-tp5845262p5845262.html Sent from the Statalist mailing list archive at Nabble.com. * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: rate ratios from xtlogit using margins - how?***From:*Maarten buis <maartenbuis@yahoo.co.uk>

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