also compared content from Wikipedia with other reference works and found that Wikipedia only has to "share its lead position in the field of natural science." The DVD version of Spring 2005 received a rather negative review by Björn Hoffmann—product manager working for the Bibliographisches Institut & F.A. Brockhaus in July 2005. In November 2005 the OpenUsability project in cooperation with the Berlin-based Relevantive AG conducted a usability test of the German Wikipedia. The study focused on finding information and included a set of recommendations to change the MediaWiki interface. In February 2006, the open usability project led a second test which focused on the experience of new editors. The reports were published in English. A second test by c't in February 2007 used 150 search terms, of which 56 were closely evaluated, to compare four digital encyclopedias: Bertelsmann Enzyklopädie 2007, Brockhaus Multimedial premium 2007, Encarta 2007 Enzyklopädie and Wikipedia. With respect to concerns about the reliability of Wikipedia, it concluded: "We did not find more errors in the texts of the free encyclopedia than in those of its commercial competitors". In December 2007, German magazine Stern published the results of a comparison between the German Wikipedia and the online version of the 15-volume edition of Brockhaus Enzyklopädie. The test was commissioned to a research institute (Cologne-based WIND GmbH), whose analysts assessed 50 articles from each encyclopedia (covering politics, business, sports, science, culture, entertainment, geography, medicine, history and religion) on four criteria (accuracy, completeness, timeliness and clarity), and judged Wikipedia articles to be more accurate on average (1.6 on a scale from 1 to 6, versus 2.3 for Brockhaus with lower = better). Wikipedia's coverage was also found to be more complete and up to date; however, Brockhaus was judged to be more clearly written, while several Wikipedia articles were criticized as being too complicated for non-experts, and many as too lengthy. In 2015, a group of young historians reviewed the Massaker von Katyn article, which was deemed "excellent" by Wikipedia authors. They pointed out more than 130 factual errors and remarked that the article completely ignores the new scientific literature. Off-line publication CD November 2004 In November 2004, Directmedia Publishing GmbH started distributing a CD-ROM containing a German Wikipedia snapshot. Some 40,000 CDs were sent to registered customers of directmedia. The price was 3 euros per CD. The display and search software used for the project, Digibib, had been