The utilized reference index is determined by "The Photographic Art Market" - a publication that has been in circulation since 1980, in which the auction results from all major auction houses are listed. The enormous popularity of photography on the art market can be seen by the growing number of publications (the catalogues 1980-1985 were less than half in size of the catalogue of 2006), as well as the continuously increasing prices.
This photographic index -contains works from different epochs and regions and is based on actual auction results. The composition of the index has not been changed since the beginning of its calculation, with a 14% yearly increase since 1976. However, using the Dow Jones Industrial Index, in the DJII, as a comparative reference, only 1/3 of the companies that were reported in the index in 1976, still exists today.