  Using a quantitative model for bipolar outflows driven by hydromagnetic
protostellar winds, we calculate the efficiency of star formation assuming that
available gas is either converted into stars or ejected in outflows. We
estimate the efficiency of a single star formation event in a protostellar
core, finding 25%-70% for cores with various possible degrees of flattening.
The core mass function and the stellar initial mass function have similar
slopes, because the efficiency is not sensitive to its parameters. We then
consider the disruption of gas from a dense molecular clump in which a cluster
of young stars is being born. In both cases, we present analytical formulae for
the efficiencies that compare favorably against observations and, for clusters,
against numerical simulations. We predict efficiencies in the range 30%-50% for
the regions that form clusters of low-mass stars. In our model, star formation
and gas dispersal happen concurrently. We neglect the destructive effects of
massive stars: our results are therefore upper limits to the efficiency in
regions more massive than about 3000 Msun.
