Understanding Identifiability in Secondary Health Data (Commentary) (Report‪)‬

Provincial administrative health records can be important sources of secondary health data for researchers. Not only do these data yield larger sample sizes, but they may also give information about health diagnoses that differ from self-report surveys (with neither being a true "gold standard"). Secondary data allow researchers to compare health across subgroups of the population, study patterns or risk factors over time and understand influences on disease that interact to produce complex effects that cannot be detected with other data. When combined with recent advances in computer technology and statistical methodology, secondary health data are a rich resource for potential research questions in a wide variety of fields. While some secondary data are released in a form that is publicly accessible, most researchers must request access to these data through data custodians in addition to going through a process of ethics review. In Canada, these secondary health data custodians typically include hospitals and provincial health ministries governed by provincial and federal privacy legislation. This legislation is often specific to health data, (1) and in the case of provincial legislation, usually covers multiple intraprovincial jurisdictions. (2) While there are national guiding principles for research ethics boards to determine ethical use of secondary data, (3,4) there are few national guidelines for data custodians to follow when assessing the suitability of releasing data for research. Part of this may be due to differences in provincial privacy legislation, (5) however, the existence of national terms of reference for the use of secondary health data (6) suggests a widespread acknowledgement of a need for national standards. Recently, we made an application to a data custodian outside our province of residence for access to small area cancer data. Our interest was in analyzing municipality-level incidence of several types of cancer for persons 65 years of age and older. In our request, the only attributes associated with each observation were year and municipality of residence. While the data we requested were temporally and geographically aggregated, small populations made it possible that some municipalities would have fewer than 5 cancer cases in some years. The data custodian refused our request on the grounds that they did not release aggregate data with such small counts. Their primary stated concern was the identifiability of individual stakeholders.