Appraisals are not simply someone making an educated guess as to value. True appraisals are based upon: comparative price research, using published international auction records for all works by the artist, dealer’s prices, lists and verbal quotations for other works by the artist, the appraiser's personal knowledge of private transactions concerning the artist’s work and market activities of collectors, dealers, and institutions interested in the artist's works. 

​​Ellery H. Kurtz, AAA - American Art Appraisal