Year	Percent
1971	27.49
1972	24.88
1973	28.86
1974	27.5
1975	24.99
1976	23.11
1977	22.17
1978	19.64
1979	17.14
1980	16.91
1981	20.39
1982	20.55
1983	19.09
1984	18.17
1985	18.67
1986	17.37
1987	17.43
1988	17.42
1989	18.83
1990	26.16
1991	34.08
1992	41.61
1993	47.08
1994	45.65
1995	47.45
1996	48.58
1997	56.79
1998	51.07
1999	22.87
2000	24.08
2001	23.42
2002	33.65
2003	21.51
2004	27.24
2005	26.34
2006	24.88
2007	23.4
2008	24.14
2009	24.82
2010	25.03