A Few Questions About Error Coin Values

1. Most people don't want key date coins that have errors...thus it almost always decreases their value.

2. No, there is no scale. Errors are valued according to their visibility (how obvious they are) and their rarity.

Terminology correction:

The CherryPickers' Guide lists very few errors. It is a die variety hunter's guide. Die varieties and errors are different, and they do not cross. If you want a good error book, you should pick up an error encyclopedia. I believe there is one by Arnie Margolis and Fred Weinberg, and another by Alan Herbert. There is also a book put out by the ANA on the Minting Process and Errors.

Doubled dies, RPMs, OMMs, repunched dates, and over dates are NOT technically classified as errors, and you will not find much if any information on them in an error guide. Conversely, errors are NOT technically classified as die varieties, thus you will not find any information about errors in a die variety guide. It's important to learn and know the difference, not only so you know what you're talking about, but know which books to get for your interest area. None of the authors cross the two in their guides, it would be wise to learn not to cross the two when looking for the right guide for you.

Good to know, Thank's
I think the term "Error" is in itself, a two way street.
I understand what you stated and it makes sense. But I can see
how many newbies can be easily confused. Everyone understands
things differently, boy do I know that:)
What might have been an Error /Mistake by Joe Schmoe at the mint
i.e.: Oooop's put that 1992 lincoln rev. die in with the 2008
obv. then went on my break.......Oh well. An error on his part, but not on the coins part.
This site is the best one I have found on line. All aspects seem to be covered and if not, there are links and books etc. that cover them.
Seems like all who answer have patience with what may seem to be the same old question over and over again. That's great though, and Good Work to those involved. I remember when I was a young boy all we had was one book to go by. And you had to get a new one every year or else? :) Maybe someone who writes a lot better than I do, can write a book about the recent history of coin collecting and how it was done etc.