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I think AngelList is not strictly speaking empirical data -- it is what a company provides as advertisement, not what it is actually paying.

How one decides "empirical data", as well as "comparable companies" and "comparable candidate" are all subject to very broad interpretation.

For instance, GlassDoor has way lower salary numbers than AngelList. But they have their share of problems too -- while the numbers are empirical, they are often dated (and have no account of equity compensation)