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Fitting and interpreting Cragg's tobit alternative using Stata


  • Burke, William J.


In this article, I introduce the user-written command craggit, which simultaneously fits both tiers of Cragg’s (1971, Econometrica 39: 829–844) “twotier” (sometimes called “two-stage” or “double-hurdle”) alternative to tobit for corner-solution models. A key limitation to the tobit model is that the probability of a positive value and the actual value, given that it is positive, are determined by the same underlying process (i.e., the same parameters). Cragg proposed a more flexible alternative that allows these outcomes to be determined by separate processes through the incorporation of a probit model in the first tier and a truncated normal model in the second. Also, tobit is nested in craggit, making the latter a popular choice among “two-tier” models. In the article, I also present postestimation syntax to facilitate the understanding and interpretation of results.

Suggested Citation

  • Burke, William J., 2009. "Fitting and interpreting Cragg's tobit alternative using Stata," Stata Journal, StataCorp LP, vol. 9(4), pages 1-9.
  • Handle: RePEc:ags:stataj:143014
    DOI: 10.22004/ag.econ.143014

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    1. Christopher F Baum, 2006. "An Introduction to Modern Econometrics using Stata," Stata Press books, StataCorp LP, number imeus, April.
    2. Cragg, John G, 1971. "Some Statistical Models for Limited Dependent Variables with Application to the Demand for Durable Goods," Econometrica, Econometric Society, vol. 39(5), pages 829-844, September.
    3. Peter Kennedy, 2003. "A Guide to Econometrics, 5th Edition," MIT Press Books, The MIT Press, edition 5, volume 1, number 026261183x.
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