is probably the most outstanding “free” parameter here) have not been explored fully. It is therefore not clear that the dominant factor governing the formation of bound stellar clusters vs. unbound associations has been identified.

It is nonetheless possible to constrain any forthcoming theories of star and cluster formation with empirical evaluations of the probability that a cluster-sized cloud of gas is able to achieve a cumulative SFE of at least the critical 20%–50%. This probability—which can equally well be interpreted as that fraction of an ensemble of massive star-forming clouds which manages to produce bound stellar systems—is referred to here as the efficiency of cluster formation. As was indicated above, an estimate of this will be taken first from a detailed investigation of the old globular cluster populations in three giant elliptical galaxies; general arguments will then be used to show that the result also applies to globular cluster formation in other galaxies, and to the formation of disk clusters today.

A first (and very rough) assessment of the formation efficiency of globular clusters appeals to the most basic and most easily measured observable of a GCS: namely, its total population, ${\cal N}_{\rm tot}$, summed over all cluster luminosities 