Year	Percent
1975	25.05
1976	26.1
1977	27.17
1978	28.6
1979	27.86
1980	27.66
1981	27.57
1982	27.89
1983	25.67
1984	24.73
1985	24.93
1986	26.23
1987	27.56
1989	25.87
1990	25.25
1991	26.04
1992	26.62
1993	26.26
1994	23.46
1995	23.53
1996	22.34
1997	23.82
1998	24.45
1999	22.69
2000	22.8
2001	22.07
2002	20.63
2003	19.77
2004	20.04
2005	20.52
2006	22.06
2007	22.95
2008	22.82
2009	22.4
2010	21.86
2011	21.75
2012	21.19
2013	21.27