  One of the foundations of science is that researchers must publish the
methodology used to achieve their results so that others can attempt to
reproduce them. This has the added benefit of allowing methods to be adopted
and adapted for other purposes. In the field of e-Science, services -- often
choreographed through workflow, process data to generate results. The
reproduction of results is often not straightforward as the computational
objects may not be made available or may have been updated since the results
were generated. For example, services are often updated to fix bugs or improve
algorithms. This paper addresses these problems in three ways. Firstly, it
introduces a new framework to clarify the range of meanings of
"reproducibility". Secondly, it describes a new algorithm, \PDIFF, that uses a
comparison of workflow provenance traces to determine whether an experiment has
been reproduced; the main innovation is that if this is not the case then the
specific point(s) of divergence are identified through graph analysis,
assisting any researcher wishing to understand those differences. One key
feature is support for user-defined, semantic data comparison operators.
Finally, the paper describes an implementation of \PDIFF that leverages the
power of the e-Science Central platform which enacts workflows in the cloud. As
well as automatically generating a provenance trace for consumption by \PDIFF,
the platform supports the storage and re-use of old versions of workflows, data
and services; the paper shows how this can be powerfully exploited in order to
achieve reproduction and re-use.
