The Department of Health has the capability to select comparison or control populations for study cohorts to facilitate case-control studies.
Controls can be selected to meet several criteria, including:
- Demographic characteristics (e.g., year of birth, sex)
- Location (postcodes, SEIFA or other geographical features)
- Clinical outcomes or characteristics (e.g., Admitted to hospital within the same year and month)
Controls are usually frequency matched, however if required can be individually matched to cases.
Data sources
For adult cases, controls are usually selected from the Electoral Roll, such that each control was a current elector during the year in which a corresponding case had their “index event”. This does not guarantee they were resident in WA at the time, but makes it more probable.
For child cases, controls can be selected from Birth Registrations or from Midwives Notifications. Postcode matching is not very reliable in these cases – the address on the midwives Notification of Case Attended Form is not necessarily the mother’s usual address and not all birth registrations have the parents’ address. The Department of Health has no means of ensuring that children selected as controls were still resident in WA on the case’s index date.
Controls may be selected from other datasets, if appropriate for the given study.
Standard Exclusions
Most often cases are excluded from being controls. Stillborn children will be excluded from controls selected from Midwives/Births unless specifically requested otherwise. It is also possible to exclude known relatives via the Family Connections System.
The Department of Health can also exclude people based on information in other datasets, e.g., exclude all women who are known to have had a hysterectomy, or exclude men who had lung cancer in a certain time period.
Control to Case Ratio
Data Applicants should advise ISPD Client Services of their required ratio, noting that a high number of controls (e.g., 10:1 or above) may require justification. In some cases, the matching criteria may need to be relaxed to find a suitable control (e.g., where a large number are requested or the criteria are highly specific).
For more information on control selection and how to apply, please contact DataServ@health.wa.gov.au