Year	Percent
1971	33.08
1972	29.82
1973	29.34
1974	30.66
1975	29.02
1976	27.06
1977	25.91
1978	24.43
1979	22.02
1980	23.33
1981	21.33
1982	18.24
1983	19.37
1984	19.35
1985	20.45
1986	22.05
1987	23.23
1988	15.3
1989	15.4
1990	14.71
1991	10.85
1992	10.72
1993	15.07
1994	15.5
1995	19.11
1996	20.35
1997	14.88
1998	15.8
1999	15.66
2000	16.38
2001	15.84
2002	16.06
2003	19.66
2004	17.67
2005	27
2006	27.37
2007	23.78
2008	26.66
2009	26.94
2010	28.17