Okay, this study was published in early 2008 but I didn’t discovered it until recently. “Comparison of Wikipedia and other encyclopedias for accuracy, breadth, and depth in historical articles” by Lycy Holman Rector published in Reference Services Review vol. 36, häfte 1, 2008, s. 7-22.
Nine Wikipedia articles, only historical entries, were compared against articles in Encyclopaedia Britannica, The Dictionary of American History and American National Biography Online to compare Wikipedia’s accuracy. and the conclusions as expressed by Holman Rector:
“Wikipedia’s accuracy rate was 80 percent compared with 95-96 percent accuracy within the other sources. This study does support the claim that Wikipedia is less reliable than other reference resources”.
Though there has to be made more comparisons in other disciplines, preferably biograhies I think, to get more results it shows that the Nature-article, comparing Wikipedia and Encyclopedia Britannica, so many pro-wikipedians refer to need more accessorial research like this.
Rememeber also that Rosenzweig, R. (2006), “Can history be open source? Wikipedia and the future of the past”, Journal of American History, Vol. 93 No. 1, pp. 117-46. found:
“…Wikipedia to be less comprehensive and detailed than American National Biography but more so than Encarta”.