  Within the next decade, ground based gravitational wave detectors are in
principle capable of determining the compact object merger rate per unit volume
of the local universe to better than 20% with more than 30 detections. We argue
that the stellar models are sensitive to heterogeneities (in age and
metallicity at least) in such a way that the predicted merger rates are subject
to an additional 30-50% systematic errors unless these heterogeneities are
taken into account. Without adding new electromagnetic constraints on massive
binary evolution or relying on more information from each merger (e.g., binary
masses and spins), as few as the $\simeq 5$ merger detections could exhaust the
information available in a naive comparison to merger rate predictions. As a
concrete example, we use a nearby-universe catalog to demonstrate that no one
tracer of stellar content can constrain merger rates without introducing a
systematic error of order $O(30%)$ at 90% confidence. More generally, we argue
that theoretical binary evolution can depend sufficiently sensitively on
star-forming conditions -- even assuming no uncertainty in binary evolution
model -- that the \emph{distribution} of star forming conditions must be
incorporated to reduce the systematic error in merger rate predictions below
roughly 40%. (Abridged)
