The purpose of control groups

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Philip Sedgwick, reader in medical statistics and medical education

¹Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK

p.sedgwick{at}sgul.ac.uk

Researchers investigated the effectiveness of a monovalent rotavirus vaccine against severe rotavirus diarrhoea in children under 2 years. A matched case-control study design was used. Cases were 323 children under 2 years admitted to hospitals in El Salvador with laboratory confirmed rotavirus diarrhoea. Controls were 969 children matched to cases for age and neighbourhood in a ratio of 3:1.1

Details of vaccination with the monovalent rotavirus vaccine were confirmed retrospectively by inspection of vaccination cards held by parents. Details about further risk factors were collected from hospital records or in interviews with parents. The researchers concluded that monovalent rotavirus vaccine was highly effective against acquiring rotavirus diarrhoea severe enough to lead to hospital admission in children under 2 years.

Which of the following statements, if any, are true for the above case-control study?

a) When recruited all participants had severe rotavirus diarrhoea
b) The control group allowed the natural epidemiology of severe rotavirus diarrhoea to be studied
c) The control group comprised active controls