Year	Percent
1960	33.978
1961	34.238
1962	34.5
1963	34.762
1964	35.025
1965	35.289
1966	35.554
1967	35.82
1968	36.087
1969	36.354
1970	36.714
1971	37.184
1972	37.657
1973	38.131
1974	38.608
1975	39.087
1976	39.568
1977	40.051
1978	40.536
1979	40.794
1980	40.782
1981	40.77
1982	40.758
1983	40.746
1984	40.734
1985	40.722
1986	40.71
1987	40.697
1988	40.685
1989	40.897
1990	41.365
1991	41.836
1992	42.308
1993	42.78
1994	43.255
1995	43.731
1996	44.208
1997	44.685
1998	45.164
1999	45.644
2000	46.126
2001	46.606
2002	47.088
2003	47.571
2004	48.054
2005	48.537
2006	49.021
2007	49.505
2008	49.989
2009	50.472
2010	50.956
2011	51.15
2012	51.05
2013	50.95
2014	50.85
2015	50.75
2016	50.65
2017	50.55