  We describe statistical methods for measuring the exoplanet multiplicity
function - the fraction of host stars containing a given number of planets -
from transit and radial-velocity surveys. The analysis is based on the
approximation of separability - that the distribution of planetary parameters
in an n-planet system is the product of identical 1-planet distributions. We
review the evidence that separability is a valid approximation for exoplanets.
We show how to relate the observable multiplicity function in surveys with
similar host-star populations but different sensitivities. We also show how to
correct for geometrical selection effects to derive the multiplicity function
from transit surveys if the distribution of relative inclinations is known.
Applying these tools to the Kepler transit survey and radial-velocity surveys,
we find that (i) the Kepler data alone do not constrain the mean inclination of
multi-planet systems; even spherical distributions are allowed by the data but
only if a small fraction of host stars contain large planet populations (> 30);
(ii) comparing the Kepler and radial-velocity surveys shows that the mean
inclination of multi-planet systems lies in the range 0-5 degrees; (iii) the
multiplicity function of the Kepler planets is not well-determined by the
present data.
