Year	Percent
1971	30.98
1972	31.23
1973	30.63
1974	30.21
1975	31.46
1976	30.14
1977	31.17
1978	30.45
1979	31.35
1980	32.73
1981	33.86
1982	29.88
1983	30.87
1984	29.64
1985	40.89
1986	38.82
1987	35.87
1988	39.01
1989	37.04
1990	43.6
1991	45.12
1992	48.85
1993	48.4
1994	43.92
1995	42.97
1996	42.88
1997	41.15
1998	40.61
1999	39.55
2000	39.17
2001	35.58
2002	34.95
2003	37.99
2004	38.55
2005	37.3
2006	35.29
2007	36.38
2008	37.82
2009	36.56
2010	36.55