Attempts to induce hosts and friends to prevent drinkers from reaching dangerous levels of alcohol impairment count on the ability to judge impairment from observable signs of physical appearance and behavior. In a study of this ability, researchers observed and recorded signs of change in behavior and physical appearance among dosed drinkers in small social groups (N=149). Signs were grouped into impairment levels corresponding to three categories of blood alcohol concentration (BAC):​ <.05% (no signs), .05% - .08%, and >.08%. Drinkers were then classified by impairment level by observers in three settings:​ small social groups (N=333), large social groups (N=480), and public establishments (N=436). A random half of the observers in each setting were given guidance in relationships of signs to impairment level. Results showed all observers to exceed chance in their classifications of drinkers, with observations in the small social groups being significantly more accurate than those in the other two groups. The effect of guidance appeared only in the small group and at .05%-.08% BAC. The authors conclude that, while people are fairly accurate in judging alcohol impairment, their accuracy is greatest and guidance most effective when their attention can be concentrated upon a few drinkers at a time.