Controls should have the following characteristics
One way of developing a control definition is first to consider the case definition proposed for the investigation. In a hypothetical outbreak of E coli O157, mainly among young children, mainly in London, during June 2010, the following case definition might be used.
Case: Resident of London aged under 10 years with faecal isolate of E. coli O157 during June 2010.
Exclusion: Travel abroad in the week before onset of illness.
1. The source population for cases is residents of London in June 2010 aged under 10 years. Controls should be representative of this source population.
2. Since E. coli is a severe infection of children, we would expect all children in London to have a similarly high chance of being detected as cases if they had this infection. However there may be variations in proportion of cases diagnosed by geographical area through variation in factors such as health seeking behaviour, primary care sampling, diagnostic facilities. This may introduce a selection bias when we come to choose controls as it will be difficult to identify this same source population. This bias will not matter unless the proportion exposed differs between cases identified for our study and those cases who remain undetected.
3. In this definition cases have been excluded if they travelled abroad in the week before onset of illness. An equivalent suitable exclusion period for controls might be travel abroad in the week before interview. However, if cases mostly arise during school term, and if controls are interviewed in the summer holidays, some controls may be excluded unnecessarily. Another option might be to exclude those who travelled abroad in June. Or, if individually matched on potential time of exposure, travel exclusion could be restricted to the dates of the week before onset of illness in the matched case.
A suitable control definition might be:
Control: Resident of London aged under 10 years during June 2010.
Exclusion: Travel abroad in the week before interview.