): "Advocates of [self-archiving] are certain that these versions differ only trivially from the publishers' versions; many of those who oppose them are equally certain that there can be major discrepancies. In a pilot study, we have examined the actual differences in a small number of such article pairs in the social sciences and in biology. Using an operational classification of the extent of error, we have determined that neither pronouncement is likely to be correct. We found numerous small differences that affect readability between open access and publishers' versions. We also found a low frequency of potentially confusing errors, but sometimes it was the publisher's and sometimes the manuscript version that was more accurate. We found two cases where errors introduced by the publisher omit technical details that are necessary to evaluate the validity of the conclusions. However, we found no error that actually affected the validity of the data or results." 