FAQ: When defining my study population, what do I need to know about time periods and dates?
When choosing time periods and dates for your study population definitions and data extractions, you need to be aware of some potential issues that may arise. The following examples highlight some of these issues:
- If the time period for your study population window and extraction window are not aligned, i.e. your study population window is from 1985 to 1990, and your extraction window is from 1988 to 1993.
In this instance, individuals who show up in the study population may not appear in selected data files in the data extraction. For example if your study population includes all individuals who used MSP services between 1985 and 1990, but the time frame you specify for the data extraction is 1988 to 1993, an individual who used MSP services in 1986 only, would be included in the study population, but would not show up in the MSP file extract from 1988 to 1993.
- If a study population is defined based solely on the demographics data in the Consolidation File, i.e. all individuals in BC over 65 years old in 2004.
The demographics portion of Population Data BC's Consolidation File is a historical data file, and thus contains demographic information on all residents of the province. In contrast, the registration portion of the Consolidation File consists of yearly files of registered users. So, for example if your study population is all individuals in BC over 65 years old in 2004, then there may be non-registered users in your study population (as they appear in the demographics file), and thus may appear in the extract, but they will not appear in the registry file.
The issues noted above do not have to be avoided when defining your study population and identifying your data extract, but you should be aware of their effect on the data that will be extracted.