  We present measurements of the star formation efficiency (SFE) in 3D
numerical simulations of driven turbulence in supercritical, ideal-MHD, and
non-magnetic regimes, characterized by their mean normalized mass-to-flux ratio
$\mu$, all with 64 Jeans masses and similar rms Mach numbers ($\sim 10$). In
most cases, the moderately supercritical runs with $\mu = 2.8$ have
significantly lower SFEs than the non-magnetic cases, being comparable to
observational estimates for whole molecular clouds ($\lesssim$ 5% over 4 Myr).
Also, as the mean field is increased, the number of collapsed objects
decreases, and the median mass of the collapsed objects increases. However, the
largest collapsed-object masses systematically occur in the weak-field case
$\mu = 8.8$. The high-density tails of the density histograms in the
simulations are depressed as the mean magnetic field strength is increased.
This suggests that the smaller numbers and larger masses of the collapsed
objects in the magnetic cases may be due to a greater scarcity and lower mean
densities (implying larger Jeans masses) of the collapse candidates. In this
scenario, the effect of a weak field is to reduce the probability of a core
reaching its thermal Jeans mass, even if it is supercritical. We thus suggest
that the SFE may be monotonically reduced as the field strength increases from
zero to subcritical values, rather than there being a discontinuous transition
between the sub- and supercritical regimes, and that a crucial question to
address is whether the turbulence in molecular clouds is driven or decaying,
with current observational and theoretical evidence favoring (albeit
inconclusively) the driven regime.
