Year	Percent
1970	25.25873
1971	25.28379
1972	24.16934
1973	25.89226
1974	25.89533
1975	27.62228
1976	25.94333
1977	24.11136
1978	24.56167
1979	24.67203
1980	25.21714
1981	24.56418
1982	24.6185
1983	22.96071
1984	23.39184
1985	25.77107
1986	24.93145
1987	24.25651
1988	21.66225
1989	20.82724
1990	19.39281
1991	21.57591
1992	21.46625
1993	22.66776
1994	22.13233
1995	22.59829
1996	22.56239
1997	21.34731
1998	21.01665
1999	22.45841
2000	21.5483
2001	19.19779
2002	17.93161
2003	18.98863
2004	19.10955
2005	18.71516
2006	18.78419
2007	17.73939
2008	19.83168
2009	19.24973
2010	19.20904
2011	19.83719
2012	21.53536
2013	25.13748
2014	26.63456
2015	27.80288
2016	28.18837
2017	28.66716