  Policies ensuring that research data are available on public archives are
increasingly being implemented at the government [1], funding agency [2-4], and
journal [5,6] level. These policies are predicated on the idea that authors are
poor stewards of their data, particularly over the long term [7], and indeed
many studies have found that authors are often unable or unwilling to share
their data [8-11]. However, there are no systematic estimates of how the
availability of research data changes with time since publication. We therefore
requested datasets from a relatively homogenous set of 516 articles published
between 2 and 22 years ago, and found that availability of the data was
strongly affected by article age. For papers where the authors gave the status
of their data, the odds of a dataset being extant fell by 17% per year. In
addition, the odds that we could find a working email address for the first,
last or corresponding author fell by 7% per year. Our results reinforce the
notion that, in the long term, research data cannot be reliably preserved by
individual researchers, and further demonstrate the urgent need for policies
mandating data sharing via public archives.
