What it affects
- That year's attendance, spend and rates aren't shown.
- Retention for that year isn't shown.
- Retention for the next year isn't shown either, because there is no reliable "last year" to compare with.
A year says Data unavailable when there aren't enough staff records for it to give real numbers. Usually that's because check-ins weren't recorded that year, or because most of that year's staff records were removed when the event was copied for the next year. Rather than show a misleading 0%, the page hides that year and says so.
Events → Reports. The row shows Data unavailable, and when you open the event a note under the table lists which years aren't shown.
What it affects