1. ¹CNR-IBIM, Clinical Epidemiology and Physiopathology of Renal Diseases and Hypertension, Reggio Calabria, Italy
2. ²ERA–EDTA Registry, Department of Medical Informatics, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands
3. ³Department of Clinical Epidemiology, Leiden University Medical Centre, Leiden, The Netherlands
4. ⁴Division of Nephrology, University Clinic, University of Würzburg, Würzburg, Germany

Correspondence: G Tripepi, CNR-IBIM, Istituto di Biomedicina, Epidemiologia Clinica e Fisiopatologia, delle Malattie Renali e dell'Ipertensione Arteriosa, c/o Euroline di Ascrizzi Vincenzo, Via Vallone Petrara n. 55/57, Reggio Calabria 89124, Italy. E-mail: gtripepi@ibim.cnr.it

Received 25 May 2007; Revised 26 July 2007; Accepted 7 August 2007; Published online 31 October 2007.

Abstract

The quality of a clinical study depends on internal and external factors. Studies have internal validity when, random error apart, reported differences between exposed and unexposed individuals can be attributed only to the exposure under investigation. Internal validity may be affected by bias, that is, by any systematic error that occurs in the design or in the conduction of a clinical research. Here we focus on two major categories of bias: selection bias and information bias. We describe three types of selection biases (incidence-prevalence bias, loss-to-follow-up bias, and publication bias) and a series of information biases (i.e. misclassification bias—recall bias, interviewer bias, observer bias, and regression dilution bias—and lead-time bias).