To find out whether living next to high-voltage power lines increases your chance of getting cancer, researchers selected various homes at random, determined if they were in 50 yards of high-voltage power line, and recorded whether anyone in home had cancer. They compared proportion of cancer cases in homes in 50 yards of high-voltage power line to proportion in home more than 50 yards from high-voltage power line. Is this observational study or experiment? Explain how do you know?