Year	Percent
1960	41.28245
1961	41.4016
1962	45.03458
1963	41.16183
1964	37.06742
1965	36.23504
1966	32.84932
1967	28.0546
1968	31.5862
1969	43.20321
1970	44.98239
1971	37.43493
1972	35.01518
1973	35.56661
1974	31.41303
1975	33.28791
1976	34.53857
1977	32.92318
1978	36.52335
1979	37.9471
1980	35.49331
1981	34.62079
1982	32.18192
1983	39.69128
1984	38.01173
1985	40.2507
1986	38.58923
1987	34.28165
1988	31.76135
1989	33.53125
1990	33.64643
1991	31.30245
1992	30.30048
1993	32.77427
1994	35.76804
1995	35.52701
1996	36.28405
1997	35.63194
1998	33.96773
1999	32.6913
2000	32.48047
2001	37.37051
2002	35.86757
2003	32.94373
2004	32.64753
2005	30.6226
2006	31.12249
2007	33.32375
2008	41.19814
2009	49.47375
2010	53.44151
2011	56.34417
2012	56.68052
2013	55.16232
2014	52.87283
2015	53.18771
2016	48.37754
2017	46.96383