Foreword TO WRITE A DICTIONARY IN ANY SCIENTIFIC DISCIPLINE IS A RISKY ENDEAVOR. BECAUSE scientists often disagree. The nature of science is not to reach consensus but to advance our knowledge by bringing conflicting ideas to critical examinations. That is true also for how we define the concepts we use. No dictionary will ever be able to satisfy all, nor should it try to he aim of the International Epidemiological Association(IEA)in cosponsor- ing this dictionary in its more than 20 years history has been to facilitate commu- nication among epidemiologists--to develop a"common language"to the extent that this is possible. We need a common language when we write papers, teach and communicate findings to the public This“ common lar nguage anges over time, as anybody can see by reading the successive editions of this dictionary. The language changes because our understanding of the concepts changes over time and new research options bring forward From the iea. we want to thank john Last for his tremendous achievements as editor of the dictionary, and we are happy to welcome Miquel Porta as the new editor. Miquel has provided the smooth transition we were looking for, and we are pleased to see that he continues the tradition of collaborating with leading epidemiologists worldwide to get the best possible result Jorn Olsen, Neil Pearce, and chitr Sitthi-Amorn Current, coming, and past presidents, International Epidemiological AssociationTo write a dictionary in any scientifi c discipline is a risky endeavor, because scientists often disagree. The nature of science is not to reach consensus but to advance our knowledge by bringing confl icting ideas to critical examinations. That is true also for how we defi ne the concepts we use. No dictionary will ever be able to satisfy all, nor should it try to. The aim of the International Epidemiological Association (IEA) in cosponsor￾ing this dictionary in its more than 20 years’ history has been to facilitate commu￾nication among epidemiologists—to develop a “common language” to the extent that this is possible. We need a common language when we write papers, teach, and communicate fi ndings to the public. This “common language” changes over time, as anybody can see by reading the successive editions of this dictionary. The language changes because our understanding of the concepts changes over time and new research options bring forward new concepts. From the IEA, we want to thank John Last for his tremendous achievements as editor of the dictionary, and we are happy to welcome Miquel Porta as the new editor. Miquel has provided the smooth transition we were looking for, and we are pleased to see that he continues the tradition of collaborating with leading epidemiologists worldwide to get the best possible result. Jørn Olsen, Neil Pearce, and Chitr Sitthi-Amorn Current, coming, and past presidents, International Epidemiological Association www.ieaweb.org Foreword v