Year	Percent
1975	45.70066
1976	47.41719
1977	47.53741
1978	47.33536
1979	47.12346
1980	46.79948
1981	47.88418
1982	48.45556
1983	48.97468
1984	49.13268
1985	50.33027
1986	51.16034
1987	51.71141
1988	50.58368
1989	50.50332
1990	52.29865
1991	55.68869
1992	56.69637
1993	56.11909
1994	55.12405
1995	54.33459
1996	55.07643
1997	54.28037
1998	53.50302
1999	53.69161
2000	52.89188
2001	54.25203
2002	54.85774
2003	55.14914
2004	55.74626
2005	55.78239
2006	54.88141
2007	54.61186
2008	56.15324
2009	59.49489
2010	58.85035
2011	59.20688
2012	60.60372
2013	60.24304
2014	60.66899
2015	60.71449
2016	60.36854
2017	60.0527