Year	Percent
1971	27
1972	21.57
1973	27.17
1974	26.99
1975	26.49
1976	29.3
1977	30.5
1978	29.79
1979	21.3
1980	18.2
1981	16.55
1982	14.26
1983	21.32
1984	27.33
1985	27.65
1986	18.55
1987	19.55
1988	20.23
1989	20.11
1990	20.8
1991	18.38
1992	18.6
1993	17.39
1994	20.1
1995	19.25
1996	21.72
1997	18.1
1998	17.52
1999	19.96
2000	23.92
2001	24.18
2002	29.23
2003	27.99
2004	25.25
2005	23.83
2006	25.62
2007	26.29
2008	20.9
2009	18.66
2010	17.77