The citation advantage of open access articles

Four subjects, ecology, applied mathematics, sociology and economics, were selected to assess whether there is a citation advantage between journal articles that have an open access (OA) version on the Internet compared to those articles that are exclusively toll access (TA). In two rounds of data collection, citations were counted using the Web of Science and the OA status of articles was determined by using the search tools OAIster, OpenDOAR, Google and Google Scholar. In the first round a purposive sample of 4633 articles for the four subjects from high impact journals were examined, 2280 (49%) were OA and had a mean citation count of 9.04, whereas the mean for TA articles was 5.76. There was a clear citation advantage for those articles that were OA as opposed to those that were TA. This advantage, however, varied between disciplines, with sociology having the highest citation advantage but the lowest number of OA articles from the sample taken and ecology having the highest individual citation count for OA articles but the smallest citation advantage. Tests of correlation between OA status and a number of variables were generally found to be weak or inconsistent but some associations were significant. Google and Google Scholar were more successful at finding OA articles on the Internet than were OAIster or OpenDOAR. The country of origin of the citing authors for applied maths was found in order to assess whether those authors from poorer countries cited OA articles more frequently than TA articles. While cited to citing article ratios from lower income countries favoured OA articles, overall percentages gave mixed results. The data from the second round confirmed the result for sociology. The second sample for ecology was randomly taken from 82 journals and exhibited a greater OA advantage. For economics, a second purposive sample of articles from 21 mid-range impact journals was taken and also exhibited a greater OA advantage. In an attempt to establish the cause of any citation advantage, logistic regression was used to try to determine whether the bibliographic characteristics of the articles from both rounds could be used to predict OA status. Results from this were generally inconclusive.