Graph of the SFST's WAT = Walk And Turn 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: WAT score on the x-axis, BAC on the y-axis. If you know a driver's WAT score, do you really know their BAC?

On innocent people, the WAT gives the wrong answer more often than it gives the right answer--it is less accurate than a coin toss.

R^2 = 37%. Only 37% of the variation in BAC can be explained by variation in WAT score. (R^2 is the "Coefficient of Determination")