[U] User's Guide

[ U ] 20.15 Obtaining conditional and average marginal effects 295. margins, dydx(south) atmeansConditional marginal effects Number of obs = 1868Model VCE : OIMExpression : Pr(union), predict()dy/dx w.r.t. : 1.southat : 0.collgrad = .7521413 (mean)1.collgrad = .2478587 (mean)0.south = .5744111 (mean)1.south = .4255889 (mean)tenure = 6.571065 (mean)Delta-methoddy/dx Std. Err. z P>|z| [95% Conf. Interval]1.south -.1236055 .019431 -6.36 0.000 -.1616896 -.0855215Note: dy/dx for factor levels is the discrete change from the base level.At the means of all the covariates, southern women are 12 percentage points less likely to bemembers of a union. This marginal effect includes both the direct effect of i.south and the interactionsouth#c.tenure.As margins reports below the table, this change in the response is for the discrete change ofgoing from not southern (0) to southern (1).The header of margins tells us where the marginal effect was estimated. This margin includes75% of the effect of college graduates and fixes tenure to be 6.6 years. There is nothing specialabout this point. We could also evaluate the marginal effect at the median of tenure:. margins, dydx(south) atmeans at((medians) _continuous)Conditional marginal effects Number of obs = 1868Model VCE : OIMExpression : Pr(union), predict()dy/dx w.r.t. : 1.southat : 0.collgrad = .7521413 (mean)1.collgrad = .2478587 (mean)0.south = .5744111 (mean)1.south = .4255889 (mean)tenure = 4.666667 (median)Delta-methoddy/dx Std. Err. z P>|z| [95% Conf. Interval]1.south -.1061338 .0201722 -5.26 0.000 -.1456706 -.066597Note: dy/dx for factor levels is the discrete change from the base level.With tenure at its median of 4.67, the marginal effect is about 2 percentage points less than itwas at the mean of 6.6.When examining conditional marginal effects, it is often useful to evaluate them at a range of valuesfor the covariates. We can do that by asking both for values of the indicator covariate collgrad andfor a range of values for tenure:(Continued on next page)