# RE: st: confidence intervals for ratio of predictions-- bootstrap vs. parametric methods?

 From "Daniel Waxman" To statalist@hsphsun2.harvard.edu Subject RE: st: confidence intervals for ratio of predictions-- bootstrap vs. parametric methods? Date Sun, 14 Oct 2007 17:03:37 -0400

```Not sure about list etiquette regarding answering one's own questions,
bootstrap that I am interested in appears to be given in the following
reference (in particular, "marginal standardization"):

Berlin JA, Margolis DJ, Localio AR.  Relative risks and confidence
intervals were easily computed indirectly from multivariable logistic
regression.  Journal of Clinical Epidemiology  2007; 60: 874-882
(the reference has appeared previously on this list).

With interaction terms, this works out to:

. global Xpos _b[_cons] + _b[int_zlog_pos]*zlog  + ///
_b[int_zero_pos]*zero + _b[zlog]*zlog  + _b[zero]*zero + _b[mbpos]

. global Xneg _b[_cons] + _b[zlog]*zlog + _b[zero]*zero

. program boot_posneg, rclass
. logistic outcome zlog zero mbpos int_z*
. tempvar rrt
. gen `rrt'=invlogit((\$Xpos))/invlogit((\$Xneg)) if e(sample)
. qui sum `rrt' ,meanonly
. return scalar rr=r(mean)
. end

. bootstrap rr_univ=r(rr), reps(1000): boot_posneg
. estat bootstrap

Which, I believe, provides the 95% CIs for the relative risk
for condition mbpos=1 vs. mbpos=0 in the population as otherwise specified.

(this is somewhat different from what I said I wanted, but it does the job).

Daniel
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```