Year	Percent
1971	18.5
1972	20.81
1973	19.45
1974	10.9
1975	13.32
1976	12.71
1977	13.52
1978	11.15
1979	11.09
1980	12.53
1981	13.93
1982	12.71
1983	11.78
1984	11.28
1985	11.54
1986	11.97
1987	10.58
1988	11.3
1989	19.21
1990	15.28
1991	14.61
1992	16.19
1993	16.46
1994	17.34
1995	17.15
1996	18
1997	17.8
1998	15.81
1999	17.45
2000	18.76
2001	20.22
2002	19.61
2003	16.87
2004	20.01
2005	21.36
2006	21.9
2007	20.87
2008	21.44
2009	23.05
2010	21.22