Year	Percent
1970	36.15385
1971	36.56716
1972	36.49635
1973	36.87943
1974	37.5
1975	37.83784
1976	37.5
1977	37.82051
1978	38.125
1979	38.41463
1980	39.28571
1981	39.30636
1982	39.54802
1983	39.56044
1984	40.32258
1985	40.31414
1986	40.81633
1987	40.79602
1988	41.26214
1989	41.70616
1990	41.93548
1991	42.34234
1992	42.73128
1993	42.91845
1994	43.09623
1995	43.26531
1996	43.8247
1997	43.96887
1998	44.48669
1999	44.81481
2000	45.28986
2001	45.58304
2002	45.86207
2003	46.12795
2004	46.38158
2005	46.94534
2006	47.16981
2007	47.54601
2008	47.90419
2009	48.3871
2010	48.7106
2011	48.88268
2012	49.45355
2013	50
2014	50.13055
2015	50.5102