Year	Percent
1970	45.88565
1971	41.95039
1972	47.2595
1973	49.14123
1974	55.07483
1975	61.14027
1976	63.70951
1977	63.2112
1978	63.25914
1979	63.63802
1980	61.21874
1981	58.56873
1982	58.82701
1983	57.30616
1984	57.713
1985	56.93285
1986	55.90439
1987	53.71675
1988	52.27785
1989	52.28905
1990	50.77844
1991	49.94581
1992	46.73992
1993	45.4905
1994	46.87359
1995	26.00212
1996	25.72725
1997	25.34594
1998	25.72877
1999	24.45829
2000	26.63542
2001	23.58239
2002	24.33294
2003	24.46451
2004	21.8298
2005	20.40412
2006	19.45007
2007	18.5063
2008	19.36626
2009	17.68481
2010	17.63618
2011	16.85281
2012	15.24398
2013	14.88047
2014	13.76306
2015	13.0375
2016	12.51473