# st: RE: adjust prevalence (solution)

 From Lee Sieswerda To "'Jan Brogger'" , Statalist Subject st: RE: adjust prevalence (solution) Date Fri, 30 Aug 2002 19:35:10 -0400

```I think the issue is that the terms "adjustment" and "standardization" are a
bit muddled up in the literature of different disciplines.
Using Jan's last data set as an example:

Jan wants to sex-adjust his asthma prevalence at time1 so that it is
comparable with the asthma prevalence at time0, which has a different sex
distribution. In epidemiology, we (stupidly) interchange the terms
"standardized" and "adjusted". What Jan is looking for is standardization.

But lets trace the problem from Jan's original attempt to use -adjust-.

After running the logistic regression (logistic asthma sex time), we obtain
the probabilities for the four possible combinations of predictors (i.e.,
covariate patterns) like so:
gen prob = exp( -2.197225 + _b[sex]*sex + _b[time]*time)/(1+exp( -2.197225
+_b[sex]*sex + _b[time]*time))
or more simply,
predict prob, pr
tab prob
prob |      Freq.     Percent        Cum.
------------+-----------------------------------
.1 |       1400       70.00       70.00
.3999999 |        600       30.00      100.00
------------+-----------------------------------
Total |       2000      100.00

With 4 covariate patterns, you would normally get four predicted
probabilities, except that Jan has contrived an example where the
sex-specific asthma rates are precisely the same at time0 and time1. Thus,
you only get two predicted probabilities.
To get the conditional probabilities, you can substitute 0's and 1's into
the above equation. For example, for males at time0, you would type:
di exp( -2.197225 + _b[sex]*1 + _b[time]*0)/(1+exp( -2.197225 +_b[sex]*1 +
_b[time]*0))
.3999999

Now, from this equation, it becomes clear, why -adjust- starts giving
results unexpected by someone who uses "standardize" interchangably with
"adjust". But first, in epidemiological terms, to "adjust" for, say, sex
using the direct standardization method, you multiply the standard sex
distribution (the distribution at time0 in this case) by the sex-specific
rates at time1, then add them together and divide by the total standard
population:
500*.1 = 5
500*.4 = 200
(200+50)/1000 = .25
Thus, .25 is the new "sex-standardized" asthma rate at time1. Put another
way, it is the asthma rate conditional on the sex distribution being the
same at time1 as at time0.
Now, consider what -adjust- does. Typing:
calculates the probability of asthma at the *average* of sex at .3:
di exp( -2.197225 + _b[sex]*.3 + _b[time]*0)/(1+exp( -2.197225 +_b[sex]*.3 +
_b[time]*0))
di exp( -2.197225 + _b[sex]*.3 + _b[time]*1)/(1+exp( -2.197225 +_b[sex]*.3 +
_b[time]*1))
giving a value of .15980265, which doesn't seem to correspond to anything
useful at all in terms of the "standardization" definition of "adjustment".
The same applies if you set sex=.5. However, this isn't surprising when you
consider that the value of _b[sex] was calculated based on the total sample
of 2000. That is not the slope that Jan wants. If you want to
"adjust/standardize" the asthma rate at time1 to the standard sex population
at time0, then maybe what you are really looking for is this:

* Get the rate in the population to be standardized
logistic asthma sex time if time==1

* Apply the slope/rate derived from the population at time=1
* to the standard population at time=0
predict prob
tab prob if time==0
sum prob if time==0

This appears to work. But just to be sure, why don't we modify the data a
little to remove the contrivance of equal sex-specific rates at time0 and
time1 and check again:

* NB: This probably won't work exactly the same for you.
* It depends on how the data are sorted. But
* what I'm doing is changing 7 values
* of asthma at time==1 from 0 to 1
replace asthma = 1 in 1176
replace asthma = 1 in 1175
replace asthma = 1 in 1177
replace asthma = 1 in 1179
replace asthma = 1 in 1180
replace asthma = 1 in 1181
replace asthma = 1 in 1182

* Now estimate the model at time==1
* and then predict to the whole sample
logistic asthma sex time if time==1
predict prob

* Now apply the slope/rate from time1 to time0
sum prob if time==0

Variable |     Obs        Mean   Std. Dev.       Min        Max
-------------+-----------------------------------------------------
yhat4 |    1000    .2538889   .1461842   .1077778         .4

* According to this, the sex-standardized rate at time1
* using the sex ratio at time0 will be .2538889
* We can double-check that this is correct using -dstdize-
gen pop = 1
dstdize asthma pop sex, by(time) base(0)

<snipped out most of output>

Summary of Study Populations:
time             N      Crude     Adj_Rate       Confidence Interval
--------------------------------------------------------------------------
0          1000   0.250000     0.250000    [  0.224824,    0.275176]
1          1000   0.137000     0.253889    [  0.204823,    0.302955]

Notice that -dstdize- has produced exactly the same standardized rate
(.253889) as the method using logistic regression above.

Whew, this problem of Jan's has been bugging me for days. I'm glad to
finally have a working solution. I'd love to see any other comments or
solutions.

Best,

Lee

Lee Sieswerda, Epidemiologist
Thunder Bay District Health Unit
999 Balmoral Street
Thunder Bay, Ontario
Tel: +1 (807) 625-5957
Fax: +1 (807) 623-2369
Lee.Sieswerda@tbdhu.com
www.tbdhu.com

> -----Original Message-----
> From:	Jan Brogger [SMTP:jan.brogger@med.uib.no]
> Sent:	Wednesday, August 28, 2002 6:03 AM
> To:	Statalist
> Cc:	geir.egil.eide@helse-bergen.no; 'Lee Sieswerda'; 'VISINTAINER PAUL'
> Subject:	adjust prevalence (solution)
>
> 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 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'
>
> 	}
>
> 	*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'
>
> 	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
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```