The NASA Astrophysics Data System: Obsolescence of Reads and Cites

The obsolescence of an article, how its use declines as it ages, has long been a central element of bibliometric studies. Normally this is determined using the citations to an article. We determine this function using the reads an article receives and then compare this with the function determined from a citation study. There are both similarities and differences. The similarities are strong enough that the normative theory of citations must be true in the mean. 1. Readership as a function of age Because the use of the Astrophysics Data System (ADS) is now the dominant means by which astronomers access the technical literature the ADS usage logs can provide a uniquely powerful view of the way an entire discipline (astronomy) uses the technical literature. Here we will examine the obsolescence (e.g. White and McCain (1989), Line and Sandison (1975)) of the technical literature of astronomy as a function of article age based on the actual readership of an article. This is an extension and reexamination of the work done in Kurtz et al. (2000). We use, as our basic data source, the log of all article “reads” using the ADS between January first and August 20th, 2001. We define a “read” as every time a user, who has access to a list of articles, their dates, journal names, titles and authors, chooses to view more information about an article. Currently 50% of these “reads” are of the abstract, 38% are of one of the forms of whole text, 8% are of the citation list, and the rest are distributed amongst the ten other options. There are more than 4.2 million “reads” in this log. For this study we extracted those for any of the three major U.S. astronomy journals The Astrophysical Journal, The Astronomical Journal, and The Publications of the Astronomical Society of the Pacific. All three of these journals have been stable over the past century, are currently among the most important astronomy journals, and have had their full text versions beginning with their first issues available on-line through ADS since well before the beginning of the