The Rarity Factor (or Scarcity Index) used on this website have been taken from Richard W. Helbock’s book “United Status Post Offices Volume III – The Upper Midwest”.  This is an excellent resource for anyone interested in collecting postal cancellations from Minnesota and the other upper mid-west states.

Rarity Factors range from 0 to 9. Zero indicates that a post office is still in operation and so a collector can easily acquire an example of cancellation from that post office. This is not to say that cancellations with a Rarity Factor of “0” have no value and a cancellation from the early 19th century can be very collectible even though the post office is still operating today. Rarity factors in the range 1 to 3 indicate Discontinued Post Offices that have closed in recent years or from post offices where large volumes of covers have survived. Rarity Factors in the range 4 – 6 are for quite rare post offices that were open for only a few years or were closed prior to the 1930’s when postmark collection became popular. Rarity Factors in the range 7 – 9 are reserved for very rare cancellations. It is quite possible that cancels with a Rarity Factor of 8 or 9 may not exist as the post offices were open for just a very short time (perhaps just a few months) or were in places that didn’t process a high volume of postal material.