Year	Percent
1970	33.33333
1971	33.49754
1972	33.65385
1973	34.2723
1974	34.24658
1975	34.82143
1976	34.78261
1977	35.16949
1978	35.68465
1979	35.48387
1980	35.82677
1981	36.15385
1982	36.84211
1983	36.99634
1984	37.14286
1985	37.41259
1986	37.88396
1987	38.33333
1988	38.31169
1989	38.73016
1990	39.00929
1991	39.39394
1992	39.94083
1993	40.17341
1994	40.67797
1995	40.88398
1996	41.23989
1997	41.68865
1998	42.01031
1999	42.31738
2000	42.85714
2001	43.13253
2002	43.63208
2003	43.87991
2004	44.24379
2005	44.59161
2006	45.14039
2007	45.45455
2008	45.96273
2009	46.24746
2010	46.62698
2011	47.27626
2012	47.61905
2013	47.94776
2014	48.44607
2015	48.83721