  The observed increase in star formation efficiency with average cloud
density, from several percent in whole giant molecular clouds to ~30 or more in
cluster-forming cores, can be understood as the result of hierarchical cloud
structure if there is a characteristic density as which individual stars become
well defined. Also in this case, the efficiency of star formation increases
with the dispersion of the density probability distribution function (pdf).
Models with log-normal pdf's illustrate these effects. The difference between
star formation in bound clusters and star formation in loose groupings is
attributed to a difference in cloud pressure, with higher pressures forming
more tightly bound clusters. This correlation accounts for the observed
increase in clustering fraction with star formation rate and with the
observation of Scaled OB Associations in low pressure environments. ``Faint
fuzzie'' star clusters, which are bound but have low densities, can form in
regions with high Mach numbers and low background tidal forces. The proposal by
Burkert, Brodie & Larsen (2005) that faint fuzzies form at large radii in
galactic collisional rings, satisfies these constraints.
