Year	Percent
1971	35.27
1972	36.05
1973	35.41
1974	35.88
1975	34.59
1976	33.48
1977	33.66
1978	30.54
1979	28.31
1980	29.59
1981	32.17
1982	31.9
1983	31.43
1984	32.21
1985	33.01
1986	31.85
1987	29.17
1988	26.54
1989	23.13
1990	19.94
1991	15.52
1992	12.41
1993	10.01
1994	8.824
1995	9.203
1996	9.431
1997	9.246
1998	9.46
1999	11.15
2000	11.83
2001	12.29
2002	13
2003	13.03
2004	12.48
2005	13.11
2006	13.22
2007	12.54
2008	12.79
2009	12.61
2010	12.61