Year	Percent
1970	11.16071
1971	11.30435
1972	11.39241
1973	11.88525
1974	11.95219
1975	12.0155
1976	12.40602
1977	12.40876
1978	12.76596
1979	13.10345
1980	13.37793
1981	13.63636
1982	13.88013
1983	14.11043
1984	14.58333
1985	15.0289
1986	15.40616
1987	15.80381
1988	16.09499
1989	16.66667
1990	17.16418
1991	17.59036
1992	18.2243
1993	18.77828
1994	19.51754
1995	20.16985
1996	20.73922
1997	21.27237
1998	21.92308
1999	22.38806
2000	22.82609
2001	23.41549
2002	23.80137
2003	24.37396
2004	24.87805
2005	25.35658
2006	25.69659
2007	26.32375
2008	26.7356
2009	27.16763
2010	27.58133
2011	28.11634
2012	28.62958
2013	29.16112
2014	29.50392
2015	30.12821