Year	Percent
1975	66.61
1976	65.51
1977	66.05
1978	64.96
1979	65.68
1980	64.6
1985	69.24
1986	69.49
1987	68.57
1988	68.63
1989	68.82
1990	67.66
1991	67.95
1992	67.38
1993	35.07
1994	67.22
1995	67.42
1996	68.86
1997	71.06
1998	72.34
1999	74.53
2000	73.26
2001	59.35
2009	61.51
2010	61.59
2011	61.52
2012	61.91
2013	63.75
2014	63.98
2015	64.16
2016	64.57
2017	64.04