Apuntes

The Stata Journal
Editor H. Joseph Newton Department of Statistics Texas A&M University College Station, Texas 77843 979-845-8817; fax 979-845-6077 jnewton@stata-journal.com Associate Editors Christopher F. Baum Boston College Nathaniel Beck New York University Rino Bellocco Karolinska Institutet, Sweden, and Univ. degli Studi di Milano-Bicocca, Italy Maarten L. Buis Vrije Universiteit,Amsterdam A. Colin Cameron University of California–Davis Mario A. Cleves Univ. of Arkansas for Medical Sciences William D. Dupont Vanderbilt University David Epstein Columbia University Allan Gregory Queen’s University James Hardin University of South Carolina Ben Jann ETH Z¨ rich, Switzerland u Stephen Jenkins University of Essex Ulrich Kohler WZB, Berlin Frauke Kreuter University of Maryland–CollegePark Stata Press Editorial Manager Stata Press Copy Editors Jens Lauritsen Odense University Hospital Stanley Lemeshow Ohio State University J. Scott Long Indiana University Thomas Lumley University of Washington–Seattle Roger Newson Imperial College, London Austin Nichols Urban Institute, Washington DC Marcello Pagano Harvard School of Public Health Sophia Rabe-Hesketh University ofCalifornia–Berkeley J. Patrick Royston MRC Clinical Trials Unit, London Philip Ryan University of Adelaide Mark E. Schaffer Heriot-Watt University, Edinburgh Jeroen Weesie Utrecht University Nicholas J. G. Winter University of Virginia Jeffrey Wooldridge Michigan State University Lisa Gilmore Jennifer Neve and Deirdre Patterson Editor Nicholas J. Cox Department of Geography Durham University South Road Durham CityDH1 3LE UK n.j.cox@stata-journal.com

Editors’ Preface
The booklet you are reading reprints 33 Stata Tips from the Stata Journal, with thanks to their original authors. We, the Journal editors, began publishing tips in 2003, beginning with volume 3, issue 4. It pleases us now to introduce them in this booklet. The Stata Journal publishes substantive and peer-reviewed articles ranging fromreports of original work to tutorials on statistical methods and models implemented in Stata, and indeed on Stata itself. The original material we have published since 2001 includes special issues such as those on measurement error models (volume 3, number 4, 2003) and simulated maximum likelihood (volume 6, number 2, 2006). Other features include regular columns on Stata (currently, “Speaking Stata”and “Mata Matters”), book reviews, and announcements of software updates. We are pleased by the external recognition that the Journal has achieved. In 2005, it was added to two of Thomson Scientific’s citation indexes, the Science Citation Index Expanded and the CompuMath Citation Index. But back to Tips. There was little need for tips in the early days. Stata 1.0 was released in 1985. Theoriginal program had 44 commands and its documentation totaled 175 pages. Stata 9, on the other hand, has more than 700 commands—including an embedded matrix language called Mata—and Stata’s official documentation now totals more than 6,500 pages. Beyond that, the user community has added several hundred more commands. The pluses and the minuses of this growth are evident. As Stata expands, it isincreasingly likely that users’ needs can be met by available code. But at the same time, learning how to use Stata and even learning what is available become larger and larger tasks. Tips are intended to help. The ground rules for Stata Tips, as found in the original 2003 statement, are laid out as the next item in this booklet. The Tips grew from many discussions and postings on Statalist, at Users Groupmeetings and elsewhere, which underscore a simple fact: Stata is now so big that it is easy to miss even simple features that can streamline and enhance your sessions with Stata. This applies not just to new users, who understandably may quake nervously before the manual mountain, but also to longtime users, who too are faced with a mass of new features in every release. Tips have come from...