Year	Percent
1960	38.31615
1961	37.93103
1962	38.30275
1963	38.51798
1964	36.16104
1965	35.32642
1966	36.98925
1967	36.02866
1968	36.80861
1969	36.79458
1970	35.93556
1971	37.05534
1972	34.4086
1973	38.95604
1974	39.21993
1975	39.6262
1976	39.95098
1977	37.15659
1978	38.00425
1979	16.52564
1980	30.25043
1981	34.72626
1982	34.35255
1983	32.73267
1984	33.93659
1985	31.27175
1986	31.80891
1987	22.90948
1988	22.57589
1989	20.74368
1990	22.32831
1991	26.63417
1992	26.68999
1993	24.71093
1994	47.42655
1995	44.14662
1996	41.63548
1997	45.87739
1998	41.73907
1999	39.93119
2000	38.37074
2001	40.51202
2002	39.28388
2003	38.89837
2004	39.39302
2005	40.92046
2006	42.53743
2007	44.45487
2008	46.65732
2009	46.97445
2010	47.93982
2011	47.89123
2012	49.19395
2013	49.73666
2014	49.79633
2015	50.20147
2016	51.85191
2017	52.42843