Year	Percent
1971	41.25
1972	37
1973	38.77
1974	35.57
1975	33.05
1976	31.59
1977	31.51
1978	30.88
1979	30.96
1980	30.18
1981	30.96
1982	30.58
1983	30.57
1984	30.27
1985	29.44
1986	29.42
1987	28.85
1988	28.46
1989	29.17
1990	29.2
1991	29.15
1992	25.83
1993	25.18
1994	23.91
1995	23.98
1996	23.58
1997	18.12
1998	17.76
1999	19.84
2000	19.93
2001	20.3
2002	20.73
2003	21.73
2004	19.83
2005	20.06
2006	21.89
2007	22.45
2008	21.46
2009	22.2
2010	21.95