Year	Percent
1970	32.14286
1971	32.43243
1972	32.70677
1973	32.96703
1974	33.45196
1975	33.56401
1976	34.00673
1977	34.42623
1978	34.71338
1979	35.29412
1980	35.54217
1981	36.07038
1982	36.46724
1983	36.84211
1984	37.46631
1985	37.69634
1986	38.16794
1987	38.86139
1988	39.27711
1989	39.81265
1990	40.5467
1991	41.15044
1992	41.72043
1993	42.46862
1994	43.17719
1995	43.76238
1996	44.48743
1997	45.17958
1998	45.75646
1999	46.38989
2000	47.17314
2001	47.75087
2002	48.39255
2003	49.08789
2004	49.67532
2005	50.31847
2006	50.9375
2007	51.60796
2008	52.25225
2009	52.80236
2010	53.40087
2011	53.97727
2012	54.7486
2013	55.28121
2014	55.79515
2015	56.42384