Year	Percent
1960	21.8102
1961	22.42428
1962	23.06894
1963	23.75906
1964	24.52287
1965	25.33572
1966	26.19526
1967	27.41269
1968	28.80222
1969	30.28661
1970	31.88946
1971	33.08648
1972	34.1757
1973	35.34631
1974	36.59417
1975	37.91396
1976	38.94358
1977	39.8804
1978	40.86023
1979	41.86716
1980	42.88555
1981	43.71359
1982	44.46016
1983	45.25656
1984	46.18508
1985	47.25128
1986	48.23966
1987	49.20739
1988	50.2089
1989	51.23119
1990	52.28864
1991	52.48761
1992	52.22578
1993	51.99468
1994	51.78902
1995	51.60417
1996	51.41879
1997	51.24251
1998	51.20741
1999	51.20978
2000	51.18246
2001	51.05217
2002	50.94845
2003	50.89073
2004	50.91238
2005	51.04599
2006	50.9362
2007	50.79075
2008	50.52142
2009	50.38045
2010	50.25255
2011	50.06915
2012	50.04926
2013	50.06632
2014	49.99953
2015	49.98559
2016	50.01237
2017	50.0526