The instances where the database results analyzed by conventional bio statistical methods differed importantly from the results in the trial presumably reflect unmeasured confounding by indication in the database studies. Thus our findings support concerns that the validity of observational studies must always be viewed with circumspection. The studies reported herein, however, suggest that the PERR technique can identify (by differing from the results with standard statistical methods) and largely correct for the effects of unmeasured confounding, when it exists. The availability in the database of previous event rates, rather than only prevalence data, permitted performance of this analysis.