and disperses into the field. The precise value of this threshold SFE depends on details of the dynamics and magnetic field in the gas cloud before its self-destruction, and on the timescale over which the massive stars dispel the gas; but various estimates place it in the range ${\rm SFE}_{\rm crit}\sim 0.2$–0.5 (e.g., Hills 1980; Mathieu 1983; Elmegreen & Clemens 1985; Verschueren 1990; Lada, Margulis, & Dearborn 1984).

A complete theory of star formation must therefore be able to anticipate the final cumulative SFE in any single piece of gas with (say) a given mass and density, and thereby predict whether or not it will form a bound cluster. No such theory yet exists. More within reach are considerations of a statistical nature, in which a [*distribution*]{} of cumulative SFEs (and, thus, the fraction of clouds that achieve ${\rm SFE}\ge {\rm SFE}_{\rm crit}$ before self-destruction) is calculated for an ensemble of star-forming clouds, in either a protogalactic or a present-day context (see, e.g., Elmegreen 1983; Elmegreen & Clemens 1985; Elmegreen & Efremov 1997). But even here, the systematic effects of a range in the initial masses of the gaseous clouds, or of changes in the assumed star formation law (which 