Study designs and their potential influence on conclusions.

Epidemiology is the basic medical science that focuses on the distribution and determinants of disease frequency in human populations. An understanding of the tools of epidemiology is helpful in defining the limitations of medical research and evaluating the conclusions of studies. This is the second in a series of three articles whose objective is to present the basic concepts of epidemiology and biostatistics, highlighted by examples of these concepts applied in the medical literature studying patients receiving dialysis. This article describes the study designs of case-control, cohort, and randomized trials and issues in the conduct and analysis of each. Several studies examining the association between hematocrit and survival of dialysis patients are discussed. Their differences in design and methods are discussed in the context of their effect on study conclusions. The goal of this article is to present the reader with the issues of study design and the limitations they impose on study conclusions. Through the examination of these methods, the reader will be able to rigorously examine study methods and understand how design affects study conclusions and their application to patient care.