The author of ERIC II, Ras Suarez, blogged on rarity: http://dirtyoldcoins.com/Roman-Coins-Blog/?m=201407 It is a difficult topic. I responded today (copied below) and long ago I wrote a long editorial about rarity for Coin World: http://esty.ancients.info/numis/rarity.html I don't expect us to come to agreement on a definition of "rare" but the relevant factors can be exposed and considered. My "comment" to the blog post is repeated here: We need to decide on a classification scheme before the term "rare" has a clear meaning. Are we talking about "all coins of some emperor" or "all coins of that emperor with this particular reverse design" or "all coins of the emperor with this particular reverse design and specific mintmark in exergue" or "all coins of the emperor with this particular reverse design and specific mintmark in exergue and marks in the field (possibly right versus left)." That is how the term "rare" can be misused without quite lying. Pick the minor variety with full details and many coins are "rare", but not necessarily as a "type" (main design, not counting field marks and minor variations). Consider the extremely common AE3 of Valentinian with GLORIA ROMANORVM reverse. Overall, it is one of the most common Roman "types" of all. But RIC lists 5 pages of mintmark and control mark variety details for the Siscia mint alone! Someone may say that, including the specific details, their coin is "rare." But that is so misleading as to be kin to a lie. So, if we are talking of the entire output of an emperor and it is only a very small fraction (say, 1%) of the output of, say, Constantine, then we might begin to think the emperor is "rare". That fraction in ERIC II would be the number (about 156) of coins for Marius, Orbiana, and Aemilan. They are rightly called "rare" with numbers far larger than those proposed in the blog. Of course, that is because rarity is relative to the category--you must decide on the category first.
