An advertisement for a lawn service claims their fertilizer treatment kills weeds in half the time. When you read the small print at the bottom of the ad, you see that they asked 20 of their long-term customers to estimate how long it took until their weeds died out before employing the lawn service. On average, the customers said it took 4 weeks for all the weeds to die before the service. The company knows that their fertilizer treatment can kill weeds in 2 weeks.