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

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

Subject |
st: adjust prevalence (solution) |

Date |
Wed, 28 Aug 2002 12:02:34 +0200 |

If you have two populations with different prevalences of an outcome, but that also have different prevalences of covariates (confounders) - how do you compare these fairly? Logistic regression will adjust for the differences, and will give you an adjusted odds ratio for the differences between populations. But how about prevalences ? The traditional method is via standardization but this cannot handle continuous variables which is a weakness. The -adjust- command and variants will adjust the prevalences so that they are comparable, but the population that it adjusts to is a not so simple meta-population with funny values for covariates [shown in previous post]. How to adjust to one of the populations ? Let prevA be the unadjusted prevalence in population A, and prevB be the unadjusted prevalence in population B. We want prevalences adjusted to population A. Naturally, the prevalence in population A is adjusted to itself, and hence unchanged. But how to get the prevalence in population B ? It seems intuitive to me that the right way is simply to use the adjusted odds ratio. Convert the unadjusted prevalence in population B to an odds, multiply by the adjusted odds ratio, and convert back to a prevalence - the adjusted prevalence. This makes sense. If you really believe that the adjusted odds ratio is the true odds ratio, due to the removal of confounding by other covarites, then the adjusted prevalence in population B represent that prevalence you would have observed if population B had the same distribution of covariates as population A. This program will calculate adjusted prevalences with the above method. It was a quick hack. Can anybody help in getting confidence intervals from this program, too ? It needs documentation (short) but I don't write SMCL. *! 1.0.0 Jan Brogger jan@brogger.no 28aug2002 capture program drop adjust2 program define adjust2 , rclass version 7.0 syntax , by(varname) coeff(string) preserve qui { tempname eb matcoeff tab `by' if `r(r)'!=2 { di as err "Error in adjust: by variable `by' must have only two levels." } *First, get the two levels of the -by- variable summ `by' local levelA `r(min)' local levelB `r(max)' *Convert the target population prevalence to an odds summ `e(depvar)' if `by'==`levelA' , meanonly local prevA=`r(mean)' local adjprevA=`prevA' local oddsA=`prevA'/(1-`prevA') *Get the unadjusted prevalence of population B summ `e(depvar)' if `by'==`levelB' , meanonly local prevB=`r(mean)' *Get the odds ratio matrix `eb'=e(b) mat `matcoeff'=`eb'["y1","`coeff'"] local coeff2=`matcoeff'[1,1] local oddsratio=exp(`coeff2') *Adjust the odds with the odds ratio and convert back local oddsB=`oddsA'*`oddsratio' local adjprevB=`oddsB'/(`oddsB'+1) } *Display this nicely di as text "Unadjusted" _col(40) "Adjusted" di "prevalences" _col(20) "Odds ratio" _col(40) "prevalences" di as res "`levelA'" _col(10) "`levelB'" _col(40) "`levelA'" _col(50) "`levelB'" di as text _dup(55) "-" di as res %4.3f `prevA' _col(10) %4.3f `prevB' _col(20) %4.3f `oddsratio' /* */ _col(40) %4.3f `adjprevA' _col(50) %4.3f `adjprevB' return local unadjprevA = `prevA' return local unadjprevB = `prevB' return local oddsratio = `oddsratio' return local adjprevA = `adjprevA' return local adjprevB = `adjprevB' restore end *Theoretical example *The adjusted prevalences should be the same *whereas the crude are not *This is due to (extreme) confounding by gender clear input time sex asthma freq 0 0 0 450 0 0 1 50 0 1 0 300 0 1 1 200 1 0 0 810 1 0 1 90 1 1 0 60 1 1 1 40 end expand freq tab sex time , col nofreq bysort time: tab asthma sex , col nofreq logistic asthma time sex *adjust sex , by(time) pr format(%16.15f) adjust2 , by(time) coeff(time) ret li Yours sincerely, Jan Brogger, 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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