The paper in the pdf file in this directory is a draft of a chapter
of Evaluating Policy Regimes, Ralph Bryant, Peter Hooper, and
Catherine Mann, editors, (Washington: Brookings Institution),
1993, p.430-443.  Thus its copyright is owned by Brookings and is
subject to the usual fair use doctrine rather than the more relaxed
conditions I have attached to most other papers on this site.  Some
parts of the paper may be unclear without reference to the rest of
the volume, but the model is of some interest in itself.