Year	Percent
1971	15.68
1972	14.59
1973	13.23
1974	13.81
1975	14.68
1976	15.16
1977	15.23
1978	15.11
1980	15.36
1981	14.15
1982	14.19
1983	13.75
1984	14.29
1985	13.83
1986	13.38
1987	14.64
1988	15.57
1989	15.77
1990	15.03
1991	16.04
1992	15.96
1993	15.52
1994	15.78
1995	15.6
1996	16.64
1997	16.71
1998	16.38
1999	16.27
2000	16.22
2001	14.57
2002	15.61
2003	14.43
2004	14.81
2005	14.61
2006	14.68
2007	14.37
2008	13.71
2009	13.4
2010	13.55
2011	13.35
2012	15.27
2013	15.58
2014	15.93
2016	17.45
2017	18.27