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From |
"Jan Brogger" <jan.brogger@med.uib.no> |

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

Subject |
st: adjust after logistic, worked example |

Date |
Sun, 21 Sep 2003 10:43:39 +0200 |

This is in re to Laural Copeland/Elisabeth Eby. Questions on -adjust- pop up every now and then. I've worked through an example. Hope it helps. In my experience, -adjust- is dangerous if used blindly, specifically when used after logistic to get adjusted percentages/prevalences. Could this be the problem? Conceptually, it is simple. You must decide which population to adjust to. But then you must tell -adjust- to what population you want to adjust. If you don't specify which population to adjust to, -adjust- will give you results that may not be what you want. The situation does get somewhat sticky if you have interactions and/or spline variables (but still solvable) A simple worked example is below. This is data from a randomized trial, but we can pretend it is an observational study. It should be analyzed with survival analysis, but let's pretend we don't need to. .sysuse cancer , clear .ds .gen alive=1-died .xi:logistic alive i.drug .adjust, by(drug) pr The use of drug is associated with a much higher survival. The unadjusted odds ratios for both drugs is 25. The observed survival fraction is 5% with placebo and 57% in both drug groups. Let's pretend that the association is confounded by age. .xi:logistic alive i.drug age The adjusted odds ratio for drug 2 is 35 and for drug 3 is 26 (a 40% and 4% change in the odds ratio, respectively). Now what about the adjusted percentage survival? You can do: .adjust, by(drug) pr Apparently, the observed survival is 4.2% in the placebo group, and 58% in both drug groups. But to what population is this adjusted ? This estimate is not adjusted for age - because you need to specify it: .adjust age, by(drug) pr To what population is this adjusted? It is to a population that has the same mean age as the combined study sample. This may not be the most interesting population to adjust to - it may not be meaningful at all. So let's adjust to the age distribution of the placebo group. .summ age if drug==1 .adjust age=56.05, by(drug) pr Now, the observed survival is 4.2% in the placebo group, and 61% in drug 2 group but 53% in drug 3 group. If you believe that age should be adjusted for, and the age distribution in the placebo group is a meaningful population to adjust to, then these are the adjusted percentages that you should. Yours Jan Brogger PhD fellow Dept. of Thoracic Medicine, Institute of Medicine, University of Bergen, Norway * * 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/

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