METHODOLOGY Epidemiology in a changing world: variation, causation and ubiquitous risk factors

We are all living in the era of globalization and, like it or not, it is going to change the way we practise epidemiology, the kinds of questions we ask and the methods we use to answer them. However, the methods, and ways of thinking about the health of populations, that will be required for epidemiology in the 21st cen-tury are in some instances quite different from the standard epi-demiological techniques that are taught in most textbooks and courses today. As we develop epidemiological methods for address-ing the scientific and public health problems of the 21st century, it is important that we consider, once again, the distinction between the analysis of variance and the analysis of causes. This has pri-marily been considered with respect to genetic research, and also with regard to the problems of making comparisons between dif-ferent populations and environments at the same point in time. It has not been considered in depth with regard to the issues of con-ducting epidemiological research in a world that is changing over time. In this article, I first consider the statistical and scientific issues involved in the distinction between the analysis of variance and the analysis of causes. I then discuss some examples of the implications of this distinction for the theory and practice of epi-demiology in a changing world, particularly with regard to risk factors that become ubiquitous over time. Sometimes the most im-portant causes of disease are invisible because they are everywhere