Quantifying the accuracy with which physical properties of asteroids can be determined from thermal modeling is critical to measuring the impact of infrared data on our understanding of asteroids. Previous work has used independently derived diameters (from asteroid radar, occultations, and spacecraft visits) to test the accuracy of the NEOWISE diameter determinations. Here, we present a new and different method for bounding the actual NEOWISE diameter errors in the Main Belt based on our knowledge of the albedos of asteroid families. We show the 1 relative diameter error for the Main Belt population must be less than 17.5% for the vast majority of objects. For a typical uncertainty on H magnitude of 0.2 mag, the relative error on diameter for the population would be ∼10%.