  This paper discusses different needs and approaches to establishing
``causation'' that are relevant in legal cases involving statistical input
based on epidemiological (or more generally observational or population-based)
information. We distinguish between three versions of ``cause'': the first
involves negligence in providing or allowing exposure, the second involves
``cause'' as it is shown through a scientifically proved increased risk of an
outcome from the exposure in a population, and the third considers ``cause'' as
it might apply to an individual plaintiff based on the first two. The
population-oriented ``cause'' is that commonly addressed by statisticians, and
we propose a variation on the Bradford Hill approach to testing such causality
in an observational framework, and discuss how such a systematic series of
tests might be considered in a legal context. We review some current legal
approaches to using probabilistic statements, and link these with the
scientific methodology as developed here. In particular, we provide an approach
both to the idea of individual outcomes being caused on a balance of
probabilities, and to the idea of material contribution to such outcomes.
Statistical terminology and legal usage of terms such as ``proof on the balance
of probabilities'' or ``causation'' can easily become confused, largely due to
similar language describing dissimilar concepts; we conclude, however, that a
careful analysis can identify and separate those areas in which a legal
decision alone is required and those areas in which scientific approaches are
useful.
