[U] User's Guide

296 [ U ] 20 Estimation and postestimation commands. margins collgrad, dydx(south) at(tenure=(0(5)25))Conditional marginal effects Number of obs = 1868Model VCE : OIMExpression : Pr(union), predict()dy/dx w.r.t. : 1.south1._at : tenure = 02._at : tenure = 53._at : tenure = 104._at : tenure = 155._at : tenure = 206._at : tenure = 25Delta-methoddy/dx Std. Err. z P>|z| [95% Conf. Interval]1.south_at#collgrad1 0 -.0627725 .0254161 -2.47 0.014 -.112587 -.01295791 1 -.0791483 .0321151 -2.46 0.014 -.1420928 -.01620382 0 -.1031957 .0189184 -5.45 0.000 -.140275 -.06611642 1 -.1256566 .0232385 -5.41 0.000 -.1712031 -.08011013 0 -.1496772 .0222259 -6.73 0.000 -.1932392 -.10611513 1 -.1760137 .0266874 -6.60 0.000 -.2283202 -.12370734 0 -.2008801 .036154 -5.56 0.000 -.2717407 -.13001964 1 -.2282 .0419237 -5.44 0.000 -.3103689 -.1460315 0 -.2549707 .0546355 -4.67 0.000 -.3620543 -.14788725 1 -.2799495 .0613127 -4.57 0.000 -.4001201 -.15977896 0 -.3097656 .0747494 -4.14 0.000 -.4562717 -.16325946 1 -.3289702 .0816342 -4.03 0.000 -.4889703 -.1689701Note: dy/dx for factor levels is the discrete change from the base level.We now have a more complete picture of the effect that being in the South has on union participation.For those with no tenure and without a college degree (the first line in the table), being in the Southdecreases union participation by only 6.3 percentage points. For those with 25 years of tenure andwith a college degree (the last line in the table), being in the South decreases participation by almost33 percentage points. We can read the effect for any combination of tenure and college graduationstatus from the other lines in the table.20.15.2 Obtaining average marginal effectsTo compute average marginal effects, the marginal effect is first computed for each observationin the dataset and then averaged. If the sample over which we compute the average marginal effectrepresents a population, then we have estimated the marginal effect for the population.We continue with our example of labor union participation.. use http://www.stata-press.com/data/r11/nlsw88(NLSW, 1988 extract). probit union i.collgrad i.south tenure south#c.tenure(output omitted )