Figure 2 shows the fitted fractional polynomial curve to the aggregated data. Values of R2 (i.e., percent variance in the outcome explained) for the models fitted to the raw data and aggregated data were .47 and .79, respectively. In the fractional polynomial regression model, the dependent variable is aggregated BAC which is modeled as a function of VSR. Therefore, in order to estimate the volume corresponding to a given BAC level, the regression equation must be solved (i.e., inverted) for volume. Note that the y-axis of the figure indicates actual BAC level but also includes a sub-axis that represents a BAC range corresponding to the estimated number of drinks (obtained from Widmark equation). These estimated drink ranges were computed by inserting the volume bin endpoints described in the analysis section.
