Marginal Effects 9.4. In some cases, such as the sample selection models, it is necessary to mix the results of two sets of estimates to obtain the marginal effects. In the second case, the means are computed for each stratum, and a full set of results is given for each stratum as well as for the overall mean.
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The stratification variable may be any variable in the data set, and need not appear in the model. An output table is produced with all of the marginal effects grouped side by side. As noted above, these are computed at the means of the full sample as well as at the means of the strata. Two models are different, however. For these models, we compute marginal effects by computing the derivatives for each observation and averaging the derivatives, rather than computing the derivatives at the means of the data.