Year	Percent
1970	29.88929
1971	28.50449
1972	25.87804
1973	25.89366
1974	24.74307
1975	23.06905
1976	22.976
1977	23.919
1978	23.75998
1979	22.87522
1980	23.74861
1981	20.81685
1982	19.7974
1983	20.66219
1984	21.25545
1985	21.5911
1986	22.84723
1987	22.75578
1988	24.13447
1989	24.90179
1990	24.3817
1991	23.467
1992	23.45804
1993	21.39089
1994	21.63885
1995	21.85276
1996	22.72762
1997	23.14576
1998	23.22978
1999	23.82038
2000	23.04039
2001	22.82985
2002	21.23231
2003	20.81114
2004	20.68294
2005	20.90691
2006	21.51233
2007	22.25492
2008	22.35095
2009	20.94601
2010	20.42036
2011	20.51984
2012	19.1559
2013	18.23476
2014	18.46961
2015	19.92444
2016	20.05562
2017	20.15841