Year	Percent
1960	18.632
1961	19.291
1962	19.969
1963	20.664
1964	21.378
1965	22.108
1966	22.856
1967	23.622
1968	24.407
1969	25.207
1970	26.025
1971	26.86
1972	27.713
1973	28.581
1974	29.47
1975	30.382
1976	31.311
1977	32.252
1978	33.209
1979	34.181
1980	35.167
1981	36.163
1982	37.173
1983	38.194
1984	39.834
1985	42.373
1986	44.956
1987	47.567
1988	50.195
1989	52.814
1990	55.422
1991	58
1992	55.019
1993	52.009
1994	48.981
1995	45.96
1996	42.964
1997	43.304
1998	43.646
1999	43.988
2000	44.331
2001	44.673
2002	45.017
2003	45.361
2004	45.706
2005	46.051
2006	46.396
2007	46.742
2008	47.088
2009	47.445
2010	47.813
2011	48.193
2012	48.583
2013	48.985
2014	49.397
2015	49.82
2016	50.254
2017	50.697