Year	Percent
1970	15.44118
1971	15.82734
1972	15.38462
1973	15.64626
1974	15.89404
1975	16.12903
1976	16.3522
1977	16.56442
1978	16.16766
1979	16.37427
1980	16.47727
1981	16.66667
1982	17.2973
1983	17.36842
1984	17.4359
1985	17.5
1986	17.56098
1987	17.61905
1988	18.13953
1989	18.09955
1990	18.58407
1991	18.53448
1992	18.90756
1993	18.85246
1994	19.2
1995	19.14062
1996	19.39163
1997	19.7026
1998	19.92754
1999	20.14134
2000	20.34483
2001	20.53872
2002	20.72368
2003	20.90032
2004	21.31661
2005	21.40673
2006	21.49254
2007	21.86589
2008	22.22222
2009	22.28412
2010	22.55435
2011	22.87234
2012	23.11688
2013	23.35025
2014	23.51485
2015	23.72881