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Sunday, April 12, 2009

More evidence that downloads predict citations

Andrew B. Watson, Comparing citations and downloads for individual articles, Journal of Vision, April 3, 2009.  Editorial.  Excerpt:

...The number of citations by other articles is at present the gold standard for evaluation of the impact of an individual scientific article. Online journals offer another measure of impact: the number of unique downloads of an article (by unique downloads we mean the first download of the PDF of an article by a particular individual). Since May 2007, Journal of Vision has published download counts for each individual article. So far as we know, we are the only scientific journal providing these numbers. In the most recent accounting in July, 2008, the top five articles were each downloaded between 1,993 and 3,478 times. While we cannot equate download of an article with actually reading it, these are nonetheless remarkable numbers. The reader may wonder how total downloads of an article compare with the more traditional measures of citation count. Elsewhere I and others have discussed the differences between, and advantages and disadvantages, of download and citation counts (Watson, 2007) (Brody, Harnad, & Carr, 2006; “Deciphering citation statistics,” 2008; Perneger, 2004). In this note, I discuss the degree of correlation between these two measures....


  1. Overall correlation between total downloads and total citations of Journal of Vision articles is 0.74.

  2. Citations and downloads increase with article age in a characteristic way, but relative to downloads, citations are delayed by about 2 years and reduced by a factor of about 45.

  3. For papers published in a single year, the correlation is as high as 0.8, and usually above 0.6.

  4. The correlation between age-normalized statistics of DemandFactor (downloads/year) and CiteFactor (citations/year) is about 0.62.

  5. Download statistics provide a useful indicator, two years in advance, of eventual citations. Downloads are also a useful measure in their own right of the interest and significance of individual articles.

PS:  Also see the journal's press release and our past posts on this correlation (1, 2).

Update (4/13/09). Also see Bill Hooker's comments.