We report the results of a double-blind study of the ability to detect Earth-mass exoplanets in multi-planet systems using a combination of the proposed astrometric space mission SIM-Lite and ongoing ground-based radial velocity (RV) observations. Three independent teams contribute to this study. Team A comprises five groups of exoplanet system modelers who each contribute over 100 theoretical systems that are consistent with current expectations of the distribution functions of mass and orbital parameters of nearby exoplanet systems. Team B takes these models as input and generates simulated data sets of SIM-Lite and RV observations, using appropriate observing constraints and expected noise levels. Team C groups analyze the simulated data sets to extract the underlying planet mass and orbital parameters. The input models depend on current theoretical estimates of the distribution functions of parameters in multi-planet systems. The results of this study have significant implications for future searches for terrestrial-mass planets.