BULLETIN A publication to promote communication among Stata users Editor Associate Editors

subscription prices are posted at www.stata.com/bookstore/stb.html. Previous Issues are available individually from StataCorp. See www.stata.com/bookstore/stbj.html for details. Submissions to the STB, including submissions to the supporting files (programs, datasets, and help files), are on a nonex-clusive, free-use basis. In particular, the author grants to StataCorp the nonexclusive right to copyright and distribute the ma-terial in accordance with the Copyright Statement below. The author also grants to StataCorp the right to freely use the ideas, including communication of the ideas to other parties, even if the material is never published in the STB. Submissions should be addressed to the Editor. Submission guidelines can be obtained from either the editor or StataCorp. Copyright Statement. The Stata Technical Bulletin (STB) and the contents of the supporting files (programs, datasets, and help files) are copyright c by StataCorp. The contents of the supporting files (programs, datasets, and help files), may be copied or reproduced by any means whatsoever, in whole or in part, as long as any copy or reproduction includes attribution to both (1) the author and (2) the STB. The insertions appearing in the STB may be copied or reproduced as printed copies, in whole or in part, as long as any cop