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Davis (2008) analyzes citations from 2004-2007 in 11 biomedical journals. 15%of authors paid to make them Open Access (OA). The outcome is a significant OAcitation Advantage, but a small one (21%). The author infers that the OAadvantage has been shrinking yearly, but the data suggest the opposite. Furtheranalyses are necessary:  (1) Not just author-choice (paid) OA but Free OA self-archiving needs to betaken into account rather than being counted as non-OA.  (2) proportion of OA articles per journal per year needs to be reported andtaken into account.  (3) The Journal Impact Factor and the relation between the size of the OAAdvantage article 'citation-bracket' need to be taken into account.  (4) The sample-size for the highest-impact, largest-sample journal analyzed,PNAS, is restricted and excluded from some of the analyses. The full PNASdataset is needed.  (5) The interaction between OA and time, 2004-2007, is based on retrospectivedata from a June 2008 total cumulative citation count. The dates of both thecited articles and the citing articles need to be taken into account.  The author proposes that author self-selection bias for is the primary causeof the observed OA Advantage, but this study does not test this or of any ofthe other potential causal factors. The author suggests that paid OA is notworth the cost, per extra citation. But with OA self-archiving both the OA andthe extra citations are free.

Confirmation Bias and the Open Access Advantage: Some Methodological Suggestions for the Davis Citation Study