  A comprehensive new approach is presented for deriving probability densities
of physical properties characterizing lens or source that constitute an
observed galactic microlensing event. While previously encountered problems are
overcome, constraints from event anomalies and model parameter uncertainties
can be incorporated into the estimate. Probability densities for given events
need to be carefully distinguished from the statistical distribution of the
same parameters among the underlying population from which the actual lenses
and sources are drawn. Using given model distributions of the mass spectrum,
the mass density, and the velocity distribution of Galactic disk and bulge
constituents, probability densities of lens mass, distance, and the effective
lens-source velocities are derived, where the effect on the distribution that
arises from additional observations of annual parallax or finite-source
effects, or the absence of significant effects, is shown. The presented
formalism can also be used to calculate probabilities for the lens to belong to
one or another population and to estimate parameters that characterize
anomalies. Finally, it is shown how detection efficiency maps for binary-lens
companions in the physical parameters companion mass and orbital semi-major
axis arise from values determined for the mass ratio and dimensionless
projected separation parameter, including the deprojection of the orbital
motion for elliptical orbits. Compared to the naive estimate based on 'typical
values', the detection efficiency for low-mass companions is increased by
mixing in higher detection efficiencies for smaller mass ratios (i.e. smaller
masses of the primary).
