Does living in the South cause high blood pressure? Data from a group of 6278 whites and blacks questioned in the Third National Health and Nutritional Examination Survey between 1988 and 1994 indicates that a greater percentage of Southerners have high blood pressure than do people in any other region of the United States (see CNN.com web site article of January 6, 2000, titled "High Blood Pressure Greater Risk in U.S. South, Study Says"). This difference in rate of high blood pressure was found in every ethnic group, gender, and age category studied. List at least two possible reasons why we cannot conclude that living in the South causes high blood pressure.