Marginal Effects and Significance Testing with Heckman’s Sample Selection Model: A Methodological Note
This paper illustrates two techniques for calculating the statistical significance of the marginal effects derived from Heckman’s sample selection model,an increasingly common econometric specification in political science. The discussion draws on an analysis by Sweeney (2003) of the incidence and intensity of interstate disputes. After replicating his results, the paper presents the delta method and the nonparametric bootstrap as alternative techniques for obtaining standard errors of the marginal effects, which themselves are calculated from a transformation of the model parameters.The analysis reveals two variables for which misleading inferences are drawn with respect to the precision of the estimated coefficients in the original study, suggesting that significance testing of the derived marginal effects is warranted.
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- Heckman, James J, 1979.
"Sample Selection Bias as a Specification Error,"
Econometric Society, vol. 47(1), pages 153-61, January.
- Papke, Leslie E. & Wooldridge, Jeffrey M., 2005. "A computational trick for delta-method standard errors," Economics Letters, Elsevier, vol. 86(3), pages 413-417, March.
- Rodolfo Hoffmann & Ana Lucia Kassouf, 2005. "Deriving conditional and unconditional marginal effects in log earnings equations estimated by Heckman's procedure," Applied Economics, Taylor & Francis Journals, vol. 37(11), pages 1303-1311.
- Atanu Saha & Oral Capps & Patrick Byrne, 1997. "Calculating marginal effects in models for zero expenditures in household budgets using a Heckman-type correction," Applied Economics, Taylor & Francis Journals, vol. 29(10), pages 1311-1316.
- Weihua Guan, 2003. "From the help desk: Bootstrapped standard errors," Stata Journal, StataCorp LP, vol. 3(1), pages 71-80, March.
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