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From |
Michael Norman Mitchell <Michael.Norman.Mitchell@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Testing equality of odds ratios outside of logistic regression |

Date |
Sun, 09 May 2010 17:13:07 -0700 |

Dear Michael

I hope this is helpful, Best regards, Michael N. Mitchell See the Stata tidbit of the week at... http://www.MichaelNormanMitchell.com ---snip--- * make fake data . sysuse auto, clear (1978 Automobile Data) . gen himpg = mpg > 20 . gen smtrunk = trunk < 14 . gen smprice = price < 5000 . . * run model 1, predicing himpg from smtrunk . logit himpg smtrunk Iteration 0: log likelihood = -51.265861 Iteration 1: log likelihood = -35.88057 Iteration 2: log likelihood = -35.860081 Iteration 3: log likelihood = -35.860072 Iteration 4: log likelihood = -35.860072

------------------------------------------------------------------------------

-------------+----------------------------------------------------------------

------------------------------------------------------------------------------ . estimates store m1 . . * run model 2, predicting himpg from smprice . logit himpg smprice Iteration 0: log likelihood = -51.265861 Iteration 1: log likelihood = -48.528905 Iteration 2: log likelihood = -48.527122 Iteration 3: log likelihood = -48.527122

------------------------------------------------------------------------------

-------------+----------------------------------------------------------------

------------------------------------------------------------------------------ . estimates store m2 . . * estimate the two models together . suest m1 m2 Simultaneous results for m1, m2

------------------------------------------------------------------------------ | Robust

-------------+---------------------------------------------------------------- m1_himpg |

-------------+---------------------------------------------------------------- m2_himpg |

------------------------------------------------------------------------------ . . * Is the coefficient for "smtrunk" equal to "smprice" . test [m1_himpg]smtrunk = [m2_himpg]smprice ( 1) [m1_himpg]smtrunk - [m2_himpg]smprice = 0 chi2( 1) = 7.68 Prob > chi2 = 0.0056 On 2010-05-09 4.49 PM, Michael I. Lichter wrote:

I'm assisting on a paper where we examine the relationship betweeneach of four dichotomous predictors variables and one dichotomousoutcome variable. Prediction is our primary objective. The predictorsare all measures of more or less the same thing, and we want to knowwhether, *without controlling for any of the others*, they predict theoutcome equally well. We want to be able to say, "If you could onlypick one of these variables as a predictor of the outcome, it wouldn'tmake any difference which one you selected."For each of the predictors we calculate a odds ratio and acorresponding confidence interval. The odds ratios are very similar inmagnitude and have confidence intervals that overlap almost entirely.We did not do any formal tests, not knowing of any offhand, and,because this isn't a central point, we didn't think it was veryimportant. When we reported that the odds ratios were essentiallyequal, a reviewer objected that we had not tested for equality. Anysuggestions?In logistic regression, by the way, two of the four variables emergedas significant predictors and two did not, controlling for the others.That is of interest, but it doesn't answer my initial question. Atleast, I don't think it does.* * 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/

* * 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/

**References**:**st: Testing equality of odds ratios outside of logistic regression***From:*"Michael I. Lichter" <MLichter@Buffalo.EDU>

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