Comic book price guies are generally monthly or yearly publications which detail the changes in the resale value of a comic over a period of time. Each collector will have his or her own preference regarding which authority to follow, but popular and respected guides currently include The Official Overstreet Comic Book Price Guide, Comics Buyer's Guide magazine, Wizard Magazine, the Comics Buyer's Guide Standard Catalog of Comic Books, and Human Computing’s ComicBase, an inventory/databasing software program. Popular online price guides include comicbookrealm.com (formerly mycomicpile.com) (free), the largest online price guide comicspriceguide.com (free and paid services), and nostomania.com (free). Updated daily, GPA for CGC Comics (GPAnalysis.com) offers online CGC pricing data gathered from online auction houses and private dealer sales. Both the online and print price guides have their own discrepancies, so a combination of both sources may have to be used to arrive at a true value.