  In this paper an analysis of the presence and possibilities of altmetrics for
bibliometric and performance analysis is carried out. Using the web based tool
Impact Story, we have collected metrics for 20,000 random publications from the
Web of Science. We studied the presence and frequency of altmetrics in the set
of publications, across fields, document types and also through the years. The
main result of the study is that less than 50% of the publications have some
kind of altmetrics. The source that provides most metrics is Mendeley, with
metrics on readerships for around 37% of all the publications studied. Other
sources only provide marginal information. Possibilities and limitations of
these indicators are discussed and future research lines are outlined. We also
assessed the accuracy of the data retrieved through Impact Story by focusing on
the analysis of the accuracy of data from Mendeley; in a follow up study, the
accuracy and validity of other data sources not included here will be assessed.
