Success in marriage has been associated with higher education and higher age. 81% of college graduates, over 26 years of age, who wed in the 1980s, were still married 20 years later. 65% of college graduates under 26, who married in the 1980s, were still married 20 years later. 49% of high school graduates under 26 years old, who married in the 1980s, were still married 20 years later. 2.9% of adults age 35–39 without a college degree divorced in the year 2009, compared with 1.6% with a college education. A population study found that in 2004 and 2008, liberal-voting states have lower rates of divorce than conservative-voting states, possibly because people in liberal states tend to wait longer before getting married. An analysis of this study found it to be misleading due to sampling at an aggregate level. It revealed that when sampling the same data by individuals, Republican-leaning voters are less likely to have a divorce or extramarital affair than Democratic-leaning voters and independents.
Question: how many years did the study last that started in 2004?
Answer: 4