This process usually involves time spans of a decade or more, as that is
the minimum duration of most policy cycles, from emergence of a
problem through sufficient experience with implementation to render
a reasonably fair evaluation of a program’s impact (Kirst and Jung 1982;
Sabatier and Jenkins-Smith 1993). A number of studies suggest that periods
of twenty to forty years may be required to obtain a reasonable
understanding of the impact of a variety of socioeconomic conditions
and to accumulate scientific knowledge about a problem (Derthick and
Quirk 1985; Baumgartner and Jones 1993; Eisner 1993).