  Many algorithms for processing probabilistic networks are dependent on the
topological properties of the problem's structure. Such algorithms (e.g.,
clustering, conditioning) are effective only if the problem has a sparse graph
captured by parameters such as tree width and cycle-cut set size. In this paper
we initiate a study to determine the potential of structure-based algorithms in
real-life applications. We analyze empirically the structural properties of
problems coming from the circuit diagnosis domain. Specifically, we locate
those properties that capture the effectiveness of clustering and conditioning
as well as of a family of conditioning+clustering algorithms designed to
gradually trade space for time. We perform our analysis on 11 benchmark
circuits widely used in the testing community. We also report on the effect of
ordering heuristics on tree-clustering and show that, on our benchmarks, the
well-known max-cardinality ordering is substantially inferior to an ordering
called min-degree.
