Year	Percent
1971	32.91
1972	32.59
1973	32.65
1974	33.49
1975	31.62
1976	31.26
1977	30.33
1978	30.05
1979	28.56
1980	28.47
1981	30.18
1982	33.14
1983	33.42
1984	33.29
1985	34.33
1986	33.35
1987	32.42
1988	31.5
1989	30.32
1990	30.88
1991	30.24
1992	29.86
1993	29.76
1994	28.99
1995	29.51
1996	28.37
1997	26.74
1998	26.63
1999	26.89
2000	29.52
2001	29.62
2002	30.82
2003	30.17
2004	28.67
2005	28.55
2006	28.58
2007	28.02
2008	23.1
2009	22.63
2010	21.2