Population and observation periods

Define entry and follow-up rules

Define time zero and the population at risk before counting events. Distinguish unique people from episodes and person-time. Death can be a competing event rather than ordinary loss to follow-up, depending on the outcome.

Flow diagram

Steps

Define source population

Declare incident or prevalent populations and people versus episodes.

Eligible population

Specify inclusion, exclusions and observation coverage without inventing counts.

Align cohort entry and time zero

Align eligibility assessment and cohort entry. Define available look-back and baseline restrictions using information available by entry; do not require future survival or an eventual outcome.

Define follow-up

Specify start, end, censoring, competing events and repeated episodes.

Specify analysis denominators

Define unique people, eligible episodes, first or recurrent events and person-time separately for each estimate. Overlapping populations are not additive partitions.

Finalize population specification

Document selection limits and retain applicable overlapping populations separately.