Year	Percent
1980	48.26
1981	49.42
1982	48.25
1983	49.6
1984	50.51
1985	51.02
1986	49.26
1987	46.38
1988	47.1
1989	46.58
1990	47.48
1991	47.28
1992	48.91
1993	50.47
1994	51.57
1995	53.51
1996	54.11
1997	55.11
1998	56.06
1999	56.51
2000	56.65
2001	60.68
2002	61.3
2003	62.23
2004	62.86
2005	62.89
2006	64.04
2007	64.26
2008	65.67
2010	67.43
2011	69.48
2012	71.33
2013	73.45
2014	71.01
2015	68.43
2016	69.02
2017	68.52