Four tiers describing what evidence exists behind a date, from the instrument itself down to a calendar reference.
Every date carries one of four tiers. They describe what evidence exists — not how confident we feel, which is a thing no reader can check.
Tier 1, Authority-sourced. The gazette, statute or ministerial order itself was read.
Tier 2, Multiple sources. Two or more independent sources state the same thing.
Tier 3, Single official source. One official document names it.
Tier 4, Secondary source. A calendar reference, not the instrument.
Tier 1 outranks Tier 2 while naming fewer sources, and the reason is worth stating plainly: the gazette is the instrument. A university, a law firm and three newspapers reporting a holiday are legitimate sources and are still not the document that created it. One authority beats several onlookers.
The tier is derived, never typed. It follows from the assurance standard and the kinds of source a row cites, so a record whose last official source is withdrawn falls to Tier 4 the moment it is — not when somebody remembers to re-label it.
Tier 4 is the largest group, and that is a statement about the corpus rather than an apology for it. A calendar reference is a real source; it is simply not the law, and the tier says which you are looking at.
The tier is the label. Assurance is the standard that decides which label a row may carry.
The tier is about the evidence. Precision is about the date. A Tier 1 date can still be an estimate.
See also assurance · precision · verified · corroborated · compiled
1,609 rows at Tier 1 · 176 at Tier 2 · 390 at Tier 3 · 1,817 at Tier 4. See what we hold.