Two or more INDEPENDENT secondary sources agree — and independent is the load-bearing word.
Corroborated means two or more independent secondary sources state the same date. It sits below verified, where the instrument itself was read, and above compiled.
Independent is doing all the work. Two newspapers reprinting one wire story are one source. Three sites paraphrasing the same encyclopedia are one source. An aggregator agreeing with our own earlier research is one piece of evidence wearing two hats, and does not qualify.
Applied honestly this tier stays small, and that is the point. A corroborated tier that filled up would mean the independence test was not being applied.
Verified means the primary instrument was read. Corroborated means several secondary sources agree about what it says.
Compiled is one source, or several that are not independent. The line between the two grades is independence, not count.
See also assurance · verified · compiled · precision
We hold this as assurance: corroborated. See what we hold.