Year	Percent
1960	38.354
1961	38.527
1962	38.628
1963	38.729
1964	38.83
1965	38.931
1966	39.033
1967	39.134
1968	39.236
1969	39.337
1970	39.439
1971	39.544
1972	40.043
1973	40.543
1974	41.046
1975	41.551
1976	42.058
1977	42.565
1978	43.075
1979	43.586
1980	44.099
1981	44.612
1982	45.127
1983	45.642
1984	46.16
1985	46.677
1986	47.195
1987	47.714
1988	48.234
1989	48.754
1990	49.274
1991	49.794
1992	50.315
1993	51.417
1994	52.704
1995	53.988
1996	55.268
1997	56.537
1998	57.8
1999	58.357
2000	58.912
2001	59.464
2002	60.014
2003	60.561
2004	61.107
2005	61.648
2006	62.187
2007	62.774
2008	63.678
2009	64.57
2010	65.452
2011	66.325
2012	67.187
2013	68.036
2014	68.874
2015	69.7
2016	70.501
2017	71.275