Year	Percent
1971	10.71
1972	10.39
1973	11.27
1974	11.53
1975	11.23
1976	10.32
1977	11.65
1978	12.08
1979	12.12
1980	13.21
1981	13.4
1982	13.72
1983	12.53
1984	13.83
1985	14.8
1986	13.96
1987	14.35
1988	14.92
1989	13.78
1990	13.55
1991	19.22
1992	23.06
1993	24.61
1994	21.51
1995	20.41
1996	21.56
1997	19.22
1998	21.78
1999	22.67
2000	22.02
2001	20.66
2002	23.16
2003	22.74
2004	21.16
2005	20.19
2006	20.02
2007	19.38
2008	19.92
2009	23.44
2010	24.5