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In a crossover design, period describes a position in follow-up, while sequence describes the order of interventions. Recording both through one ambiguous field can make derivation and interpretation difficult, especially when actual conduct differs from the planned schedule.
Define the question and assumptions
Record assigned sequence, actual intervention, period start and measurement window separately. Disease course or measurement practices can change between periods. When one intervention is mostly observed earlier and another later, period changes can mix with the treatment comparison. Prespecify an analytical approach matching allocation and the observed data structure rather than treating all repeated observations interchangeably.
Hypothetical study example
In a hypothetical study, a delayed second-period measurement is labelled as a first-period repeat. The team examines dates, administration records and assigned sequence. It applies the documented period-mapping rule instead of choosing whichever classification produces a more attractive intervention difference in the resulting analysis.
Design review checks
- Distinguish sequence and period variables explicitly within the data model.
- Assess discrepancies between planned and actually received interventions for each period.
- Test missing, delayed and repeated assessments using representative participant examples.
Record the decision and its limits
Accepted derivation shows each observation relationship to period, intervention and sequence. An incomplete second period should not disappear without its reason and implications being reported. Period differences are not automatically treatment effects. Explain design advantages alongside carryover and follow-up limitations, avoiding certainty unsupported by the available observation structure.
Frequently asked questions
- Are sequence and period the same concept?
- No. Sequence is intervention order; period is the follow-up position.
- Should a person with a missing second period be deleted?
- Not automatically. Assess the planned analysis and reasons for missingness.