The following excerpt is published courtesy of DLRC Press and its author, David Lawrence This information was originally published in 1991 in The Complete Guide to Barber Halves
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As every coin collector knows, scarcity is not always consistent with original mintage figures. Over the years some dates get lost, melted or saved more than others. The first and last years of a mint (1906-D, 1909-0) tend to be preserved in higher grades as do the first and last dates of the series.
A date’s relative scarcity is indicated by its “Rarity Rating,” which varies from R1 to R8. Factors considered in estimating scarcity are the original mintage, the populations of certified coins (PCGS & NGC), the historical value, the number of times the date appears on customer “want lists” and the results of a survey made by the BCCS among its members.
The RARITY RATINGS are defined as follows:
R2 —A better date. Available at most shows, but in limited quantity.
R3 —A tough date. Only a few likely to be found at larger shows.
R4 — Scarce. May or may not be available at larger shows.
R5 —Very Scarce. Only a few will appear at large shows or auctions in a yeah time.
R6 — Extremely Scarce. Almost never available.
R7 — Rare. Only a few exist.
R8 — Unique or almost so.