Year	Percent
1970	22.83951
1971	22.89157
1972	22.94118
1973	23.42857
1974	23.46369
1975	23.91304
1976	23.80952
1977	24.2268
1978	24.1206
1979	24.5098
1980	24.40191
1981	24.65116
1982	25
1983	25.22124
1984	25.43103
1985	25.63025
1986	25.81967
1987	26
1988	26.17188
1989	26.23574
1990	26.7658
1991	26.81159
1992	27.20848
1993	27.24138
1994	27.60943
1995	27.96053
1996	28.20513
1997	28.52665
1998	28.74618
1999	28.95522
2000	29.15452
2001	29.62963
2002	29.80501
2003	30.16304
2004	30.58511
2005	30.64935
2006	30.96447
2007	31.26551
2008	31.5534
2009	32.06651
2010	32.32558
2011	32.72727
2012	32.88889
2013	33.33333
2014	33.6887
2015	34.02923