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One Leg Stand

This divided attention test consists of the instruction phase and the balance & counting phase. During the instruction phase, the person must stand with their feet together, keeping their arms at their side while listening to the instructions. One's attention is divided between keeping their balance and listening to and remembering the instructions.

During the balance & counting phase, the person must raise one leg approximately 6 inches off the ground, arms at their side, toes pointed out and looking at the pointed toes, and keeping both legs straight. While looking at their toes, they are to count 1001, 1002, 1003, etc. until they are told to stop. The test is to last for 30 seconds and the officer is supposed to time the test. During the test, if at anytime a person puts their foot down, they are instructed to pick it back up and continue counting where they left off.

The officer is looking for 4 specific clues:

1. sways while balancing;
    
2. using arms for balance;
    
3. hopping; and
    
4. putting foot down.

The original research has shown that if 2 or more clues are present (i.e. putting foot down 1 time and having a couple seconds of sway), the person is intoxicated. This test has been shown to be accurate 65% of the time; therefore, it is inaccurate 35% of the time! Why?