Year	Percent
1971	16.57
1972	18.54
1973	18.42
1974	18.13
1975	17.33
1976	17.56
1977	17.33
1978	16.11
1979	16.72
1980	18.09
1981	16.46
1982	17.55
1983	16.9
1984	16.61
1985	18.99
1986	19.47
1987	18.76
1988	18.14
1989	16.96
1990	17.76
1991	21.73
1992	20.01
1993	20.03
1994	19.64
1995	19.85
1996	17.41
1997	16.17
1998	16.58
1999	15.94
2000	16.72
2001	15.81
2002	18.75
2003	22.24
2004	25.03
2005	23.96
2006	17.73
2007	15.81
2008	16.87
2009	16.36
2010	17.26