The approach taken in calculating the Rarity Factor (or Scarcity Index) used on this website has been taken from Richard W. Helbock’s book “United Status Post Offices Volume III – The Upper Midwest”.  The Post Office information has been taken from Robin M. Startup's book "New Zealand Post Offices", which is an excellent resource for anyone interested in collecting postal cancellations from New Zealand.

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.