A group of researchers in africa find a creative way to protect cattle from lion attacks - they paint eyes on the cows' rears. To determine if this treatment is effective, they randomly assign the cattle to one of three treatments: eyes on their rears, cross-marks on their rear, or nothing on their rear. After 4 years of roaming the plains, the cows with eyes saw no deaths, the cows with a cross suffered 4 deaths, and the cows with no marks suffered 15 deaths