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Studying people who start an intervention addresses a different experience from studying long-standing users. The first record in a database may not be the first use in real life, so observation history and the definition of initiation need assessment together.
Define the question and assumptions
Define the sources and period available for identifying prior use. New system entry, insurance changes or care elsewhere can hide earlier exposure. Measure baseline covariates in a pretreatment window rather than carrying future information backwards. Explain how requirements for observable history affect the population that can enter the comparison.
Hypothetical study example
In a hypothetical study, someone newly entering a database is called a new user because their first prescription is recorded. Previous use elsewhere is unknown. The team limits the definition to first observed use or requires sufficient observable history instead of claiming to know a true initiation date the source cannot establish.
Design review checks
- Specify the prior non-use window and the records available to assess it.
- Distinguish interruptions in database coverage from evidence of actual non-use.
- Check that baseline characteristics are measured before the exposure initiation date.
Record the decision and its limits
Accepted reporting presents the initiation definition, observable history and selected population together. A longer history requirement can systematically exclude some people, so discuss representation consequences. Do not treat inadequate history as confirmed absence of prior treatment. Inclusion and exclusion rules should remain reproducible at the individual-record level.
Frequently asked questions
- Is the first database record the first real use?
- Not necessarily. Earlier exposure may be elsewhere or outside observation.
- Is a longer history requirement always better?
- It can improve information while also changing selection and representation.