Newson, RRNewson2015-10-302015-10-30http://hdl.handle.net/10044/1/27167© 2004 Stata Corporation. The Stata Journal and the contents of the supporting files (programs, datasets, and help files) are copyright by StataCorp LP. 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 Stata Journal. This paper is made available by permission of Stata CorporationJournal Article