  With over 20 million records, the ADS citation database is regularly used by
researchers and librarians to measure the scientific impact of individuals,
groups, and institutions. In addition to the traditional sources of citations,
the ADS has recently added references extracted from the arXiv e-prints on a
nightly basis. We review the procedures used to harvest and identify the
reference data used in the creation of citations, the policies and procedures
that we follow to avoid double-counting and to eliminate contributions which
may not be scholarly in nature. Finally, we describe how users and institutions
can easily obtain quantitative citation data from the ADS, both interactively
and via web-based programming tools.
  The ADS is available at http://ads.harvard.edu.
