In a manner that is anecdotical; based on or by means of anecdotes rather than systematic evidence.
From anecdotical (the rare adjective form) plus the adverbial suffix -ly. This creates a doubly-suffixed adverb form that is now quite archaic.
This word represents linguistic redundancy—we have anecdotally, anecdotally, and anecdotically all meaning essentially the same thing, yet only one became standard, showing how natural selection works in language.
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