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From |
"Garth Rauscher" <garthr@uic.edu> |

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

Subject |
st: model-based standardization |

Date |
Mon, 4 Feb 2008 22:11:39 -0600 |

[I tried to send this message to the listserv a few days ago but don't think it made it through so I am trying again. I apologize if this is a duplicate message.] Dear listserve members I am attempting to learn how to perform a model-based standardization with Stata, using the marginal or predictive margins method. I would like to be able to estimate standardized probabilities and probability differences from logistic regression that are standardized to the distribution of modeled covariates. The idea is summarized in: "Greenland S. Model-based estimation of relative risks and other epidemiologic measures in studies of common outcomes and in case-control studies. Am J Epidemiol 2004;160:301-305." To the best of my understanding, the method involves estimating predictied probabilities of Y under two scenarios (e.g. x=1 and x=0). Assuming we have a dependent variable Y(0,1), an exposure of interest X(0,1), and covariates r2 r3 a2 a3 a4 a5, two sets of predicted probabiltiies could be: P0(x) based on the joint distribution of covariates, with X=0 assigned to everyone P1(x) based on the joint distribution of covariates, with X=1 assigned to everyone PD(x) as the difference in probabilities, P1(x) - P0(x) Below is my code. logit Y X r2 r3 a2 a3 a4 a5 // predicted xbetas after assigning all observations to X=0 g if0=_b[_cons]+_b[X]*0+_b[r2]+_b[r3]+_b[a2]+_b[a3]+_b[a4]+_b[a5] // predicted xbetas after assigning all observations to X=1 g if1=_b[_cons]+_b[X]*1+_b[r2]+_b[r3]+_b[a2]+_b[a3]+_b[a4]+_b[a5] // predicted probabilities g p0x = invlogit(if0) g p1x = invlogit(if1) groups p0x p1x +---------------------------------------+ | p0x p1x Freq. Percent | |---------------------------------------| | .1368349 .4431594 444 100.00 | +---------------------------------------+ I was expecting two new variables of predictied probabilities, p0x and p1x with a range of values that depended on covariates. However, I noticed that p0x and p1x each had only one value instead of a range of values as I had expected (see above). Any clarification as to what I am doing incorrectly would be appreciated. I think my next task would have been to perform bootstrapping to get confidence intervals from the distribution of means for p0x, p1x and PD(x). Thanks in advance. Garth Garth Rauscher Division of Epid/Bios (M/C 923) UIC School of Public Health 1603 West Taylor Street Chicago, IL 60612 ph: (312)413-4317 fx: (312)996-0064 em: garthr@uic.edu * * 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**:**Re: st: model-based standardization***From:*"Austin Nichols" <austinnichols@gmail.com>

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