Introduction

The purpose of this article is to provide a brief overview of the range of study designs used to address research questions in clinical epidemiology. For readers with a particular research question in mind, comparison of the different options may guide selection of an appropriate study design. Clinical epidemiology can be defined as the investigation and control of the distribution and determinants of disease. Some of the other epidemiological terms used in the article are described more fully in table 1. Later articles in this series will deal with different study designs in more detail.

The range of clinical and epidemiological studies in sexual health is very wide, but in terms of purpose and basic study design, they can be divided into a few broad categories. The purpose may be to determine the level (prevalence) of disease in a population, to identify causes of disease or those at high risk of disease, to describe the natural history of disease, to prevent the onset of disease or alter the course of disease in individuals or populations. The simplest categorisation in terms of study design is between observational studies and intervention studies (trials). Observational studies, in which one observes the course of a disease or the relation between risk factors (exposures) and outcomes, are used to address questions about prevalence, natural history, aetiology, and risk groups. Trials, in which one intervenes to prevent or change the course of a disease, are used to evaluate preventive or therapeutic interventions, but can also provide strong evidence of causality (table 2).

Observational studies

Observational studies include cross sectional, cohort, case-control, and ecological studies.

CROSS SECTIONAL STUDIES

In a cross sectional study, individuals with a defined disease, risk factor, or other condition of interest are identified at a point in …