An article in Journal of Nutrition (Vol. 130, No. 8) noted that chocolate is rich in flavonoids. The article reports that "regular consumption of foods rich in flavonoids may reduce the risk of coronary heart disease." The study received funding from Mars, Inc., the candy company, and the Chocolate Manufacturers Association.
Identify and explain at least one source of bias in the study describe. Then suggest how the bias might have been avoided.