In the United States vitamin E supplement use by female health professionals was 16.1% in 1986, 46.2% in 1998, 44.3% in 2002, but decreased to 19.8% in 2006. Similarly, for male health professionals, rates for same years were 18.9%, 52.0%, 49.4% and 24.5%. The authors theorized that declining use in these populations may have due to publications of studies that showed either no benefits or negative consequences from vitamin E supplements. Within the US military services, vitamin prescriptions written for active, reserve and retired military, and their dependents, were tracked over years 2007-2011. Vitamin E prescriptions decreased by 53% while vitamin C remained constant and vitamin D increased by 454%. A report on vitamin E sales volume in the US documented a 50% decrease between 2000 and 2006, with a cause attributed to a  meta-analysis that had concluded that high-dosage vitamin E increased all-cause mortality.
Question: How many years did a 50% decrease of vitamin E sales volume happen?
Answer: 6