The internal validity of an epidemiological study can be affected by random error and systematic error. Random error reflects a problem of precision in assessing a given exposure-disease relationship and can be reduced by increasing the sample size. On the other hand, systematic error or bias reflects a problem of validity of the study and arises because of any error resulting from methods used by the investigator when recruiting individuals for the study, from factors affecting the study participation (selection bias) or from systematic distortions when collecting information about exposures and outcomes (information bias). Another important factor which may affect the internal validity of a clinical study is confounding. In this article, we focus on two categories of bias: selection bias and information bias. Confounding will be described in a future article of this series.

Keywords:
Bias, Information bias, Selection bias, Systematic error
1.
Rothman KJ: Epidemiology. An Introduction. New York, Oxford University Press, 2002, pp 20–21.
2.
Last J (ed): A Dictionary of Epidemiology, ed 3. Oxford, Oxford University Press, 1988.
3.
Tripepi G, Jager KJ, Dekker FW, Zoccali C: Measures of effect in epidemiological research. Nephron Clin Pract 2010;115:c91–c93.
4.
Noordzij M, Dekker FW, Zoccali C, Jager KJ: Measures of disease frequency: prevalence and incidence. Nephron Clin Pract 2010; 115:c17–c20.
5.
Tsai SY, Tseng HF, Tan HF, Chien YS, Chang CC: End-stage renal disease in Taiwan: a case-control study. J Epidemiol 2009;19:169–176.
6.
Stel VS, Zoccali C, Dekker FW, Jager KJ: The randomized controlled trial. Nephron Clin Pract 2009;113:c337–c342.
7.
Ganguli M, Lutle ME, Reynolds MD, Dodge HH: Random versus volunteer selection for a community-based study. J Gerontol A Biol Sci Med Sci 1998;53:M39–M46.
8.
Hernan MA, Hernandez-Diaz S, Robins JM: A structural approach to selection bias. Epidemiology 2004;14:615–625.
9.
Hessol NA, Schwarcz S, Ameli N, Oliver G, Greenblatt RM: Accuracy of self-reports of acquired immunodeficiency syndrome and acquired immunodeficiency syndrome-related conditions in women. Am J Epidemiol 2001;153:1128–1133.
10.
Tripepi G, Jager KJ, Dekker FW, Zoccali C: Bias in clinical research. Kidney Int 2008;73:148–153.
© 2010 S. Karger AG, Basel
2010
Copyright / Drug Dosage / Disclaimer
Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher.
Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug.
Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.
You do not currently have access to this content.