September 2007, Volume 18, Issue 5,pp.523-652 Original Article 523 A Polygon-Based Locally-Weighted-Average Method for Smoothing Disease Rates of Small Units. Xun Shi; Eric Duell; Eugene Demidenko; Tracy Onega; Benjamin Wilson; David Hoftiezer 529 Estimating the Effects of Time-Varying Treatments: Incidence of Fractures Among Postmenopausal Japanese Women. Shiro Tanaka; Yutaka Matsuyama; Masataka Shiraki; Yasuo Ohashi 537 Estimating the Longitudinal Prevalence of Diarrhea and Other Episodic Diseases: Continuous Versus Intermittent Surveillance. Wolf-Peter Schmidt; Stephen P. Luby; Bernd Genser; Mauricio L. Barreto; Thomas Clasen 544 Potential Confounding by Exposure History and Prior Outcomes: An Example From Perinatal Epidemiology. Penelope P. Howards; Enrique F. Schisterman; Patrick J. Heagerty 552 The Importance and Role of Intracluster Correlations in Planning Cluster Trials. John S. Preisser; Beth A. Reboussin; Eun-Young Song; Mark Wolfson 561 Four Types of Effect Modification: A Classification Based on Directed Acyclic Graphs. Tyler J. VanderWeele; James M. Robins Commentary 569 FREE Can DAGs Clarify Effect Modification? Clarice R. Weinberg Original Article 573 Are Girls More Susceptible to the Effects of Prenatal Exposure to Tobacco Smoke on Asthma? Jouni J. K. Jaakkola; Mika Gissler 577 Prenatal Ultrasound Scanning and the Risk of Schizophrenia and Other Psychoses. Karin Stålberg; Bengt Haglund; Ove Axelsson; Sven Cnattingius; Christina M. Hultman; Helle Kieler Commentary 583 FREE Belated Concerns and Latent Effects: The Example of Schizophrenia. Michaeline Bresnahan; Ezra Susser