Year	Percent
1971	11.87
1972	12.31
1973	12.13
1974	13.84
1976	21.8
1977	21.19
1978	18.64
1979	17.72
1981	20.41
1982	23.33
1983	26.52
1984	23.79
1985	24.75
1986	22.6
1987	20.32
1988	18.25
1990	17.56
1991	18.8
1992	19.09
1993	19.38
1995	16.04
1996	14.96
1997	14.91
1998	14.41
1999	13.52
2000	11.93
2002	13.37
2003	12.77
2004	11.47
2005	12.22
2006	11.21
2007	11.96
2008	12.83
2009	17
2010	18.9
2011	18.2
2012	16.2
2015	13.8