Are the findings of heterogeneity in outcomes on the basis of site enrollment troublesome? It is quite possible that the improved outcomes seen at high-enrolling sites are predominately due to the enrollment of lower-acuity patients. For this reason, further studies are necessary from other clinical trials to determine whether similar relationships are present, because the interplay between high volume, high quality, and improved outcomes has been shown in many clinical settings (8). It remains possible that the differences shown between high- and low-enrolling sites might be due to the actual quality of sites participating in the clinical trial. Just as variability exists between providers, there is considerable variability in the quality of sites participating in clinical trials. Underreporting of safety and efficacy endpoints, inadequate concomitant therapy, and high patient discontinuation rates are just a few examples of recent site-specific issues seen in trials. These issues were the impetus for recent Food and Drug Administration guidance providing support for flexible monitoring plans that are able to adjust and provide further monitoring support and oversight to sites that seem to be lagging in quality. Yet, to date, there remains no standardized way in which to measure the quality of a research site in general.