Year	Percent
1971	32.20231
1972	32.94253
1973	33.75271
1974	35.21237
1975	34.54375
1976	37.57108
1977	36.57085
1978	38.03224
1979	37.66118
1980	39.31671
1981	39.06316
1982	38.7725
1983	38.81956
1984	40.16365
1985	40.6295
1986	40.34567
1987	41.20945
1988	41.63641
1989	41.31252
1990	41.97404
1991	41.60029
1992	43.26051
1993	44.15037
1994	44.53368
1995	44.25318
1996	43.10353
1997	39.69947
1998	42.29806
1999	40.05809
2000	39.0282
2001	41.92078
2002	43.18946
2003	41.95348
2004	44.12995
2005	43.52941
2006	43.03122
2007	46.77823
2008	47.39825
2009	49.43247
2010	50.90125
2011	49.78472
2012	49.63846
2013	49.26393
2014	49.93392
2015	50.30748
2016	50.15533