Year	Percent
1971	17.94
1972	14.54
1973	11.12
1974	12.64
1975	24.54
1976	12.99
1977	14.44
1978	15.46
1979	14.89
1980	16.77
1981	15.54
1982	19.65
1983	18.67
1984	19.84
1985	22.3
1986	16.88
1987	17.36
1988	18.57
1989	20.23
1990	16.63
1991	17.24
1992	19.53
1993	24.44
1994	26.51
1995	24.42
1996	25.28
1997	26.1
1998	24.73
1999	21.23
2000	21.9
2001	21.9
2002	23.82
2003	22.4
2004	21.77
2005	19.75
2006	20.18
2007	20.27
2008	19.35
2009	20.11
2010	19.49