What is the purpose of an art price index?

Art price indices are instruments used by auction houses, art collectors and historians to provide a guide as to the proper price of a piece or style of artwork. These indices compile purchase data from auction sales worldwide and create a market index to determine a piece's sales potential.

Famous art price indices such as Mei Moses and ArtIndex publish different indices that represent different collecting categories with information gathered from major auction houses such as Sotheby's and Christie's. Many art critics contend that price indices are fundamentally flawed because they only include data from public sales and not from private gallery and dealer sales.