A study last month in Nature showed that the decision is far from clear-cut. Calling on experts to compare 42 competing entries, the journal counted an average of four errors per article in Wikipedia -- and three in Britannica. That is not much of a difference, and a look at the details only adds to the anxiety. A fact is surely a fact, but what constitutes an error can be as hard to pin down as a bead of mercury.

A high school student looking for information on Dmitri Mendeleyev (also spelled Mendeleev), the Russian chemist renowned for the periodic table of the elements, would have learned from Wikipedia that he was the 14th child in his family instead of the 13th surviving child of 17 -- what Nature's reviewer, Michael Gordin, a Princeton University science historian, said was one of 19 mistakes in the article.

But it wouldn't have helped to defer to the competition: Dr. Gordin gave Britannica a demerit for describing the chemist simply as the 17th child. It is an imprecision one might easily commit. Dr. Gordin was surprised when I told him, in an e-mail message, that his own book, "A Well-Ordered Thing: Dmitrii Mendeleev and the Shadow of the Periodic Table," uses the same number. "That's curious," he said. "I believe that is a typographical error in my book. Mendeleyev was the final child, that is certain, and the number the reliable sources have is 13."

These, he said, are in Russian, and they apparently were not consulted by "The Norton History of Chemistry," by William H. Brock, which like Wikipedia says there were 14 children, or "The Development of Modern Chemistry," in which Aaron J. Ihde goes with 17. In his book "Galileo's Finger: The 10 Great Ideas of Science," Peter Atkins, an Oxford University chemist, says that the number, "according to one's source," is 11, 14 or 17.

Wikipedia seems determined to try them all. Scrolling through the various versions of its article -- more than 300 at last count extending back to July 5, 2002 -- one can watch as the number oscillates between 14 and 17, stopping briefly at 15 (with the explanation from an anonymous editor that "a child was recentely [sic] discovered to exist") then to 16 before returning to 14 again.