Year	Percent
1970	18.53
1971	15.22
1972	15.82
1973	17.62
1974	18.58
1975	17.74
1977	19.13
1978	20.29
1979	21.63
1980	20.08
1981	21.95
1982	21.86
1983	21.25
1984	20.11
1989	20.72
1990	20.68
1991	24.84
1992	21.47
1993	21.68
1994	21.49
1995	21.89
1996	23.36
1997	22.24
1998	22.26
1999	22.58
2000	21.29
2001	21.26
2002	20.56
2003	19.75
2004	19.96
2005	21.48
2006	22.05
2007	22.13
2008	22.85
2009	23.73
2010	25.34
2011	23.52
2012	24.86
2013	24.11
2014	24.41
2015	25.08
2016	25.46
2017	26.56