Year	Percent
1960	35.679
1961	37.682
1962	39.73
1963	41.056
1964	41.585
1965	42.114
1966	42.647
1967	43.181
1968	43.717
1969	44.336
1970	45.052
1971	45.77
1972	46.491
1973	47.211
1974	47.933
1975	48.656
1976	49.381
1977	50.103
1978	50.827
1979	51.488
1980	52.083
1981	52.676
1982	53.268
1983	53.86
1984	54.452
1985	55.04
1986	55.628
1987	56.215
1988	56.8
1989	57.078
1990	57.033
1991	56.987
1992	56.942
1993	56.896
1994	56.851
1995	56.805
1996	56.76
1997	56.714
1998	56.669
1999	56.623
2000	57.133
2001	58.222
2002	59.304
2003	60.378
2004	61.443
2005	62.494
2006	63.535
2007	64.564
2008	65.581
2009	66.58
2010	67.567
2011	67.963
2012	68.03
2013	68.097
2014	68.163
2015	68.23
2016	68.297
2017	68.363