the Greenlight and early access models, and games supporting virtual reality technology. Although Steam provides direct sales data to a game's developer and publisher, it does not provide any public sales data or provide such data to third-party sales groups like NPD Group. In 2011, Valve's Jason Holtman stated that the company felt that such sales data was outdated for a digital market, since such data, used in aggregate from other sources, could lead to inaccurate conclusions. Data that Valve does provide cannot be released without permission because of a non-disclosure agreement with Valve. Developers and publishers have expressed the need to have some metrics of sales for games on Steam, as this allows them to judge the potential success of a title by reviewing how similar games had performed. This led to the creation of algorithms that worked on publicly available data through user profiles to estimate sales data with some accuracy, which led to the creation of the website Steam Spy in 2015. Steam Spy was credited with being reasonably accurate, but in April 2018, Valve added its new privacy settings that defaulted to hiding user game profiles by default, stating this was part of compliance with the General Data Protection Regulation (GDPR) in the European Union. The change broke the method Steam Spy had collected data, rendering it unusable. A few months later, another method had been developed using game achievements to estimate sales with similar accuracy, but Valve shortly changed the Steam API that reduced the functionality of this service. Some have asserted that Valve used the GDPR change as a means to block methods of estimating sales data, though Valve has since promised to provide tools to developers to help gain such insights that they say will be more accurate than Steam Spy was. In 2020, Simon Carless revised an approach originally proposed by Mike Boxleiter as early as 2013, with Carless's method used to estimate sales of a game based on the number of reviews it has on Steam based on a modified "Boxlieter number" used as a multiplication factor. In November 2011, CD Projekt, the developer of The Witcher 2: Assassins of Kings, revealed that Steam was responsible for 200,000 (80%) of the 250,000 online sales of the game. Steam was responsible for 58.6% of gross revenue for Defender's Quest during its first three months of release across six digital distribution platformscomprising four