A sample of 20 weight lifters is tested to see how much weight they can bench press. Ten of them are chosen at random as the treatment group. They participate in a special training program for three weeks. The remaining 10 are the control group. They follow their usual program. At the end of three weeks, all 20 weight lifters are tested again and the increases in the amounts they can lift are recorded. The two samples are the increases in the amounts lifted by the treatment group and the control group.