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From |
"Newson, Roger B" <r.newson@imperial.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: Binomial regression |

Date |
Sun, 5 Aug 2007 17:15:53 +0100 |

Two suggestions re binomial regression: Suggestion 1: For what it's worth, confidence intervals for risk differences (in some cases) can be reported using the -somersd- package, downloadable from SSC using the -ssc- command. Given 2 binary (0,1) variables x and y, the user can type somersd x y, transf(z) tdist and get a confidence interval for the risk difference Pr(y==1|x==1) - Pr(y==1|x==0) This method has the advantage (compared to -binreg-, -glm- etc) of using the Normalizing and variance-stabilizing hyperbolic arctangent or z-transformation, recommended by Edwardes (1995) for the general Somers' D for binary X-variates (including the special case where the y-variate is also binary). If there is a categorical confounding variable w, then the user can type somersd x y, transf(z) tdist wstrata(w) and get a confidence interval for a within-strata risk difference for pairs of observations with the same value of w. The user can alternatively specify multiple categorical confounding w-variables, and/or w-variables which specify propensity groups based on a propensity score for x==1 calculated from multiple confounding variables. Suggestion 2: To output confidence intervals for baseline odds with confidence intervals for odds ratios, the user can specify a baseline variate of ones, and then enter it into the model with the -noconst- option. For instance, the user can type: gene byte baseline=1 logit y baseline x, noconst or This trick can also be used with geometric means and their ratios. See Newson (2003). I hope this helps. Roger References Edwardes, M. D. d. B. 1995. A confidence interval for Pr(X < Y) − Pr(X > Y) estimated from simple cluster samples. Biometrics 51: 571–578. Newson R. 2003. Stata tip 1: The eform() option of regress. The Stata Journal 3(4): 445. Download post-publication update from http://www.imperial.ac.uk/nhli/r.newson/papers.htm Roger Newson Lecturer in Medical Statistics Respiratory Epidemiology and Public Health Group National Heart and Lung Institute Imperial College London Royal Brompton campus Room 33, Emmanuel Kaye Building 1B Manresa Road London SW3 6LR UNITED KINGDOM Tel: +44 (0)20 7352 8121 ext 3381 Fax: +44 (0)20 7351 8322 Email: r.newson@imperial.ac.uk Web page: www.imperial.ac.uk/nhli/r.newson/ Departmental Web page: http://www1.imperial.ac.uk/medicine/about/divisions/nhli/respiration/popgenetics/reph/ Opinions expressed are those of the author, not of the institution. -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Maarten buis Sent: 04 August 2007 07:47 To: statalist@hsphsun2.harvard.edu Subject: Re: st: Binomial regression --- Marcello Pagano <pagano@hsph.harvard.edu> wrote: > I agree wholeheartedly that the risk difference is sometimes > preferable to the odds ratio. Witness what is currently going on with > the attack on Avandia. Rather than report a risk difference of 0.2% > in the MI rate, we are faced with a risk INCREASE of 40% -- the > effect of going from 0.5% to 0.7%. If reported as a risk difference > it would probably not have made the headlines it has nor created the > furor it has. At this point I think that there is room for improvement in Stata output. When reporting odds ratios after -logit-, Stata will not report the baseline odds (-exp(_cons)-), So Stata reports that the odds increased with 40%, but not that the baseline odds is .005 (at these low probabilities risks and odds are almost the same). I would like to see the baseline odds and the odds ratios, because both give very useful information about the size of the effect, as Marcello's example illustrates. -- Maarten ----------------------------------------- Maarten L. Buis Department of Social Research Methodology Vrije Universiteit Amsterdam Boelelaan 1081 1081 HV Amsterdam The Netherlands visiting address: Buitenveldertselaan 3 (Metropolitan), room Z434 +31 20 5986715 http://home.fsw.vu.nl/m.buis/ ----------------------------------------- ___________________________________________________________ Yahoo! Answers - Got a question? Someone out there knows the answer. Try it now. http://uk.answers.yahoo.com/ * * 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/ * * 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/

**References**:**Re: st: Binomial regression***From:*Marcello Pagano <pagano@hsph.harvard.edu>

**Re: st: Binomial regression***From:*Maarten buis <maartenbuis@yahoo.co.uk>

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