A workflow is a collection of steps that must be executed in some specific
order to achieve an objective. A computerised workflow management system may
enforce authorisation policies and constraints, thereby restricting which users
can perform particular steps in a workflow. The existence of policies and
constraints may mean that a workflow is unsatisfiable, in the sense that it is
impossible to find an authorised user for each step in the workflow and satisfy
all constraints. In this paper, we consider the problem of finding the "least
bad" assignment of users to workflow steps by assigning a weight to each policy
and constraint violation. To this end, we introduce a framework for associating
costs with the violation of workflow policies and constraints and define the
\emph{valued workflow satisfiability problem} (Valued WSP), whose solution is
an assignment of steps to users of minimum cost. We establish the computational
complexity of Valued WSP with user-independent constraints and show that it is
fixed-parameter tractable. We then describe an algorithm for solving Valued WSP
with user-independent constraints and evaluate its performance, comparing it to
that of an off-the-shelf mixed integer programming package
