The reference period matters
Historical years may include unusual outbreaks, changes in testing, or disrupted care. Including or excluding them changes the expected range. That choice should be documented rather than selected to produce a preferred alarm.
Population size and reporting coverage can also change between years.
Check comparability before modeling
| Feature | Question |
|---|---|
| Calendar | Are weeks aligned consistently? |
| Case definition | Was the same event counted? |
| Reporting base | Did participating sites change? |
| Measurement | Are counts, rates, or proportions being compared? |
Expected does not mean desirable
A recurring level of illness can still be an important public-health burden. Being within a historical range does not imply that prevention is unnecessary or that individual risk is negligible.
An exceedance is a signal
Going above a baseline can prompt investigation, but it does not establish a cause. Data errors, changed testing, and real transmission changes are among the possibilities.
Use the baseline to identify departures from a defined reference, then combine it with other evidence. A transparent comparison is more useful than a dramatic “above normal” label whose normal period and calculation remain hidden.
