Year	Percent
1960	20.23049
1961	20.49968
1962	20.77132
1963	20.98285
1964	21.17034
1965	21.35638
1966	21.54289
1967	21.72999
1968	21.92453
1969	22.13054
1970	22.35535
1971	22.59897
1972	22.85855
1973	23.07498
1974	23.2884
1975	23.49161
1976	23.68523
1977	23.86693
1978	24.03711
1979	24.19164
1980	24.33081
1981	24.45147
1982	24.56044
1983	24.72941
1984	24.90308
1985	25.0815
1986	25.27033
1987	25.46921
1988	25.68282
1989	25.9106
1990	26.15877
1991	26.42826
1992	26.7211
1993	27.24656
1994	27.83985
1995	28.45998
1996	29.10986
1997	29.78718
1998	30.50103
1999	31.25437
2000	32.05333
2001	32.89286
2002	33.78084
2003	34.73351
2004	35.75131
2005	36.8303
2006	37.98248
2007	39.20521
2008	40.49384
2009	41.07115
2010	41.65664
2011	42.24532
2012	42.84128
2013	43.44287
2014	44.06028
2015	44.6964
2016	45.35174
2017	46.0256