A Maximum Likelihood Estimate (MLE): is the most accurate Maximum Likelihood Estimate is, by definition, the mode of a data set (i.e., the most frequent observation). When data are too limited to identify a clear mode, the average or the median of the data are usually substituted. For some air toxics for which adequate human data exist, EPA has based the Unit Risk Estimate on the Maximum Likelihood Estimate for response data or for fitted curves.