Year	Percent
1971	36.02
1972	35.12
1973	34.3
1974	33.52
1975	32.8
1976	33
1977	31.47
1978	31.03
1979	29.05
1980	28.58
1981	28.76
1982	30.76
1983	28.77
1984	28.95
1985	28.88
1986	30.31
1987	31.85
1988	30.99
1989	29.14
1990	28.03
1991	29.38
1992	28.71
1993	28.42
1994	28.67
1995	29.36
1996	29.39
1997	29.7
1998	29.71
1999	29.65
2000	28.04
2001	28.32
2002	29.06
2003	28.04
2004	27.9
2005	27.89
2006	27.82
2007	27.74
2008	27.42
2009	27.92
2010	26.92