Year	Percent
1960	16.96606
1961	17.75292
1962	17.78256
1963	18.25309
1964	19.18762
1965	18.53611
1966	19.33707
1967	20.26861
1968	21.19723
1969	21.24546
1970	21.92223
1971	21.06886
1972	21.28965
1973	22.77138
1974	24.0813
1975	22.03845
1976	21.75675
1977	22.77469
1978	24.60613
1979	26.4071
1980	27.58728
1981	26.44278
1982	25.24375
1983	24.97218
1984	27.98848
1985	27.59384
1986	27.02965
1987	26.03624
1988	26.07603
1989	25.05463
1990	25.07211
1991	24.33726
1992	26.17989
1993	29.13211
1994	32.77919
1995	36.10504
1996	37.10916
1997	38.14318
1998	40.04481
1999	41.8772
2000	44.23747
2001	42.0362
2002	40.0344
2003	36.82784
2004	37.28795
2005	36.79511
2006	35.32935
2007	34.15317
2008	34.32314
2009	28.43575
2010	29.07196
2011	30.56462
2012	30.2136
2013	30.16334
2014	31.5369
2015	31.52797
2016	30.96788
2017	30.89403