Macular degeneration is a serious disease of the eyes that can lead to blindness. A recent study suggests that nutrients such as lutein and zeaxanthin, found in green leafy vegetables, can slow the onset and progress of this disease. In the study, a group of older individuals were asked about their eating habits. The group who ate green leafy vegetables fewer than two times a week had a significantly higher rate of macular degeneration than those who ate green leafy vegetables five or more times per week. They also had a higher rate of cases that were considered severe among those who had the disease. (a) Is this an observational study or a controlled experiment? Explain.