Year	Percent
1971	36.89
1972	37.08
1973	35.77
1974	35.7
1975	45.78
1976	42.79
1977	40.51
1978	40.48
1979	38.08
1980	39.45
1981	38.71
1982	38.11
1983	27.48
1984	27.37
1985	29.96
1986	31.53
1987	31.08
1988	28.68
1989	27.12
1990	26.1
1991	25.77
1992	23.43
1993	20.95
1994	18.4
1995	16.06
1996	14.17
1997	14.33
1998	15.91
1999	15.04
2000	15.18
2001	15.18
2002	13.88
2003	13.22
2004	13.14
2005	13.18
2006	13.01
2007	13.59
2008	13.93
2009	13.34
2010	13.34