Nature (Number 438, 900-901 (15 December 2005) | doi:10.1038/438900a) has an interesting article comparing the frequency of errors in both Wikipedia and Britannica—the error rate is similar. Basically, 42 entries from each of the sources were given to “experts” who conducted a blind “accuracy” test.
“…The exercise revealed numerous errors in both encyclopaedias, but among 42 entries tested, the difference in accuracy was not particularly great: the average science entry in Wikipedia contained around four inaccuracies; Britannica, about three.”