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davidcyclebackRare is an absolute. Ala '10 or fewer is rare.' Though what numbers constitute rare (10 or fewer,
25 or fewer, other) is a matter of opinion, and may vary from person to person.
Scarcity is relative, demand relative to supply. An okay though imperfect measure of scarcity
is market price, as this measures supply versus demand. As scarce requires demand, an item can
be rare but not scarce (one exists but no wants it), and an item can be scarce but not rare (10,000
exist but 2 million people want them).
In collecting slang, scarce usually refers to something that is not rare, but limited in supply and tough
to get-- at least tougher to get compared closely related items. The 1974 Topps 'Washington NL' error cards
aren't rare, but are tougher to find than the San Diego versions and other singles, and the market prices reflect this.
Thus, they are commonly called scarce-- at least in the context of 1974 Topps.