[U] User's Guide

[ U ] 20.14 Obtaining marginal means, adjusted predictions, and predictive margins 291We are not restricted to margining over a single factor variable. Let’s see if the pattern of highblood pressure over age groups differs for men and women. We do that by specifying the interactionof sex and agegrp to margins.. margins sex#agegrpPredictive margins Number of obs = 10351Model VCE : LinearizedExpression: Pr(highbp), predict()Delta-methodMargin Std. Err. z P>|z| [95% Conf. Interval]sex#agegrp1 1 .0597492 .0102037 5.86 0.000 .0397502 .07974821 2 .0743686 .0090047 8.26 0.000 .0567197 .09201751 3 .139588 .0127725 10.93 0.000 .1145543 .16462181 4 .2027485 .0181733 11.16 0.000 .1671295 .23836751 5 .2154491 .0127246 16.93 0.000 .1905093 .24038881 6 .1562754 .0203574 7.68 0.000 .1163757 .19617522 1 .0178699 .0048167 3.71 0.000 .0084293 .02731042 2 .0324522 .0060014 5.41 0.000 .0206897 .04421462 3 .08906 .0129904 6.86 0.000 .0635994 .11452072 4 .1452838 .017242 8.43 0.000 .1114901 .17907752 5 .1657727 .0123653 13.41 0.000 .1415372 .19000812 6 .1833488 .0210316 8.72 0.000 .1421277 .22457Each line in the table corresponds to holding both sex and agegrp to fixed values while usingthe observed level of bmi to evaluate the probability and then averaging over the observations in thesample. To calculate the results in the first line of the table, margins fixed sex = 1 and agegrp = 1,evaluated the probability for each observation, and then averaged the probabilities. All these marginsreflect the observed distribution of bmi in the sample.The first six lines represent males, and the second six lines represent females. Comparing maleswith females for the same age groups, males are over three times as likely to have high blood pressurein the first age group (0.060/0.018 = 3.3), they are over twice as likely in the second age group,and while the relative gap narrows, it is not until above age 70 that the probability for males dropsbelow the probability for females.Can the pattern of probabilities be affected by controlling one’s bmi? Let’s reevaluate the probabilitieswhile holding bmi to two levels—20 (which is well within the normal range) and 30 (whichis at the boundary between overweight and obese). We add the option at(bmi=(20 30)) to set bmifirst to 20 and then to 30.(Continued on next page)