Study populations and extracts
A study population is the group of subjects that the Researcher wishes to include in their analyses. The subjects usually share at least one common characteristic, which may be as simple as age, or more complex, such as the set of people over the age of 65 with a certain health condition. For example, a cohort may be defined as:
- All residents of BC in calendar year 1999
- All residents of BC aged 65 years and over in calendar year 1999
- All residents of BC aged 65 years and over diagnosed with Type 2 diabetes in calendar year 1999
- All residents of BC aged 65 years and over diagnosed with Type 2 diabetes who also receive treatment for kidney failure in 1999
The study population is not the same as the data extract. An extract is the actual data requested for a study population that will help to answer a research question. For example, a researcher may be interested in the number and average length of hospital stays of all individuals in a study population, or in their use of physician services over the preceding two years.
A project may also include more than one study population for comparative purposes, similar to cohort and control groups.
Defining the study population
In the DAR, the Researcher is requested to provide specific and detailed study population information in addition to a rationale or justification for the study population(s) requested.
Data Access Unit staff at Population Data BC (PopData) work with Researchers to define the study population for a project. This interaction is necessary to ensure the desired study population can be technically produced by PopData programming staff (or external programmers, depending on the details of the project). In addition to the text description, a technical definition is also required, but will be produced in most cases by Data Access Unit staff as a result of this interaction.
For example, a Researcher might want to look at the use of health services by adolescents over a certain time period. The specifics of this could be very simple, for example, all individuals in BC aged between 13 and 18 in calendar year 2000. A more complex example might be a study population defined as all heart attack patients who received bypass surgery in calendar year 1995. The specifics required here are:
- How is a heart attack defined?
- If isolating the condition requires diagnostic codes in the hospital separations file, what are the specific codes?
- Does the code need to appear as the first (primary) diagnosis?
- How is bypass surgery defined?
- If it requires procedure codes, what are the specific codes?
- If it requires the use of MSP fee item codes, what are those fee item codes?
Data Access Unit staff at PopData then work with the Researcher to complete the technical definition for the study population(s), seeking confirmation with the Researcher. This support is also built into the process for the DAR which requires that the technical definition be completed by PopData prior to DAR submission. The Data Steward(s) use both the text and the technical definitions in their adjudication.
Researchers will download and complete the appropriate data file checklist(s) and select the fields to be used for analysis.
If a project involves more than one study population each may require a different set of data to be extracted. In this case, separate data extract checklist(s) for each study population may be completed, as applicable.
It is important to remember that the Researcher needs to request the data for their extract, and not the data required to construct a study population, when filling out the data file checklists.