Graph of the SFST's OLS = One Leg Stand test accuracy
This is a graph of the never-before-available-to-the-public data in the NHTSA's 1998 San Diego validation study. Each point represents one driver in the study: OLS score on the x-axis, BAC on the y-axis. If you know a driver's OLS score, do you really know their BAC?

R^2 = 20%. Only 20% of the variation in BAC can be explained by variation in OLS score. (R^2 is the "Coefficient of Determination")