The NASA Astrophysics Data System (ADS), along with astronomy's journals and
data centers (a collaboration dubbed URANIA), has developed a distributed
on-line digital library which has become the dominant means by which
astronomers search, access and read their technical literature. Digital
libraries such as the NASA Astrophysics Data System permit the easy
accumulation of a new type of bibliometric measure, the number of electronic
accesses (``reads'') of individual articles. We explore various aspects of this
new measure. We examine the obsolescence function as measured by actual reads,
and show that it can be well fit by the sum of four exponentials with very
different time constants. We compare the obsolescence function as measured by
readership with the obsolescence function as measured by citations. We find
that the citation function is proportional to the sum of two of the components
of the readership function. This proves that the normative theory of citation
is true in the mean. We further examine in detail the similarities and
differences between the citation rate, the readership rate and the total
citations for individual articles, and discuss some of the causes. Using the
number of reads as a bibliometric measure for individuals, we introduce the
read-cite diagram to provide a two-dimensional view of an individual's
scientific productivity. We develop a simple model to account for an
individual's reads and cites and use it to show that the position of a person
in the read-cite diagram is a function of age, innate productivity, and work
history. We show the age biases of both reads and cites, and develop two new
bibliometric measures which have substantially less age bias than citationsComment: ADS bibcode: 2005JASIS..56..111K This is the second paper (the first
is Worldwide Use and Impact of the NASA Astrophysics Data System Digital
Library) from the original article The NASA Astrophysics Data System:
Sociology, Bibliometrics, and Impact, which went on-line in the summer of
200