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Open Access Research article

Selection bias: neighbourhood controls and controls selected from those presenting to a Health Unit in a case control study of efficacy of BCG revaccination

Odimariles MS Dantas1*, Ricardo AA Ximenes23, Maria de Fatima PM de Albuquerque45, Ulisses R Montarroyos5, Wayner V de Souza5, Patrícia Varejão2 and Laura C Rodrigues6

Author Affiliations

1 Department of Mother and Child Health, Federal University of Pernambuco. Recife, Brasil, Bloco A do Hospital das Clínicas. Av. Moraes Rego s/n. Cidade Universitária, Recife, PE. Brasil

2 Department of Tropical Medicine, Federal University of Pernambuco. Recife, Brasil, Bloco A do Hospital das Clínicas. Av. Moraes Rego s/n. Cidade Universitária. Recife, PE. Brasil

3 Department of Internal Medicine, State University of Pernambuco. Recife, Brasil, Núcleo de Pós-Gaduação. Rua Arnóbio Marques 310, Campus Universitário. Santo Amaro. Recife, PE, Brasil

4 Department of Internal Medicine, Federal University of Pernambuco. Recife, Brasil, Bloco A do Hospital das Clínicas. Av. Moraes Rego s/n. Cidade Universitária. Recife, PE. Brasil

5 Research Center Aggeu Magalhães, Fundação Oswaldo Cruz. Recife, Brasil, Av. Moraes Rego s/n. Cidade Universitária. Recife, PE. Brasil

6 London School of Hygiene and Tropical Medicine, University of London. London, UK, Department of Epidemiology and Population Health. London School of Hygiene and Tropical Medicine. Room 258b, Keppel Street. London WC1E 7HT UK

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BMC Medical Research Methodology 2007, 7:11  doi:10.1186/1471-2288-7-11

Published: 23 February 2007

Abstract

Background

In most case control studies the hardest decision is the choice of the control group, as in the ideal control group the proportion exposed is the same as in the population that produced the cases.

Methods

A comparison of two control groups in a case control study of the efficacy of BCG revaccination. One group was selected from subjects presenting to the heath unit the case attended for routine prevention and care; the second group was selected from the neighbourhood of cases. All Health Units from which controls were selected offered BCG revaccination. Efficacy estimated in a randomized control trial of BCG revaccination was used to establish that the neighbourhood control group was the one that gave unbiased results.

Results

The proportion of controls with scars indicating BCG revaccination was higher among the control group selected from Health Unit attenders than among neighbourhood controls. This excess was not removed after control for social variables and history of exposure to tuberculosis, and appears to have resulted from the fact that people attending the Health Unit were more likely to have been revaccinated than neighbourhood controls, although we can not exclude an effect of other unmeasured variables.

Conclusion

In this study, controls selected from people presenting to a Health Unit overrepresented exposure to BCG revaccination. Had the results from the HU attenders control group been accepted this would have resulted in overestimation of vaccine efficacy. When the exposure of interest is offered in a health facility, selection of controls from attenders at the facility may result in over representation of exposure in controls and selection bias.