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...

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...

The Appalachian Regional Commission's definition of the Appalachian region is the one used most often by scholars, politicians, and the popular press. The uncritical use of this definition of Appalachia raises issues of both selection bias and excess heterogeneity in regression analysis of...