Year	Percent
1971	30.99
1972	32.79
1973	31
1974	33.87
1975	36.9
1976	35.31
1977	34.58
1978	30.34
1979	26.18
1980	42.72
1981	38.13
1982	24.32
1983	26.28
1984	30.42
1985	31.06
1986	35.35
1987	34.5
1988	31.8
1989	24.14
1990	23.93
1991	24.37
1992	18.01
1993	39.23
1994	40.83
1995	40.21
1996	36.55
1997	40.93
1998	27.16
1999	29.15
2000	29.45
2001	31.81
2002	31.36
2003	24.93
2004	30.33
2005	26.42
2006	25.89
2007	25.5
2008	24.75
2009	36.44
2010	25.71