Year	Percent
1971	17.37
1972	19.14
1973	18.51
1974	18.46
1975	19.23
1976	19.71
1977	19.72
1978	20.15
1979	23.4
1980	22.46
1981	23.16
1982	24.47
1983	25.86
1984	26
1985	25.98
1986	24.52
1987	23.79
1988	23.84
1989	23.92
1990	22.82
1991	23.29
1992	26.09
1993	26.8
1994	28.53
1995	29.72
1996	30.67
1997	31.01
1998	30.84
1999	30.62
2000	32.09
2001	31.24
2002	33.35
2003	32.72
2004	32.87
2005	32.05
2006	32.31
2007	31.63
2008	29.72
2009	29.93
2010	29.7