Sources of variation in biomarkers.

Epidemiology is interested in variation. The goal of epidemiological research is to infer cause-effect relationships by observing whether the occurrence of disease varies according to relevant exposures. Epidemiology is usually interested in 'intergroup' variation (e.g. between those who are exposed and those who are unexposed to the factor of interest), while 'intragroup' variation is a source of noise. Therefore, the study design aims to increase intergroup variability, to limit intragroup variability and to reduce error by ensuring validity and reliability of measurements. The goal of this chapter is to summarize simple ways to recognize the main sources of measurement error and to estimate the extent and the impact of such error.