Year	Percent
1960	6.88
1961	7.435
1962	8.031
1963	8.671
1964	9.357
1965	10.09
1966	10.874
1967	11.712
1968	12.606
1969	13.554
1970	14.564
1971	15.636
1972	16.773
1973	17.971
1974	19.237
1975	20.569
1976	21.971
1977	23.327
1978	24.627
1979	25.975
1980	27.371
1981	28.81
1982	30.295
1983	31.822
1984	33.391
1985	34.994
1986	36.634
1987	38.305
1988	39.57
1989	39.447
1990	39.323
1991	39.199
1992	39.075
1993	38.952
1994	38.829
1995	38.705
1996	38.582
1997	38.459
1998	38.337
1999	38.214
2000	38.091
2001	38.616
2002	39.48
2003	40.351
2004	41.229
2005	42.111
2006	42.998
2007	43.89
2008	44.788
2009	45.686
2010	46.588
2011	47.493
2012	48.4
2013	49.306
2014	50.203
2015	51.089
2016	51.962
2017	52.824