Year	Percent
1970	25.4717
1971	25.68807
1972	25.89286
1973	26.08696
1974	26.27119
1975	26.44628
1976	27.41935
1977	27.55906
1978	27.48092
1979	27.61194
1980	27.53623
1981	28.36879
1982	28.27586
1983	28.18792
1984	28.75817
1985	28.66242
1986	29.19255
1987	29.09091
1988	29.5858
1989	29.88506
1990	29.77528
1991	30.05464
1992	30.48128
1993	30.72917
1994	30.96447
1995	31.18812
1996	31.40097
1997	31.60377
1998	31.65138
1999	31.83857
2000	32.31441
2001	32.47863
2002	32.91667
2003	32.92683
2004	33.33333
2005	33.59073
2006	33.58491
2007	33.94834
2008	34.17266
2009	34.38596
2010	34.93151
2011	35.11706
2012	35.29412
2013	35.78275
2014	35.82555
2015	36.17021