Year	Percent
1971	33.71
1972	30.84
1973	30.54
1974	30.41
1975	31.1
1976	30.71
1977	32.88
1978	32.36
1980	32.78
1981	34.36
1982	33.72
1983	34.22
1984	35.59
1985	36.55
1986	36.84
1987	37.56
1988	38.27
1989	39.08
1990	39.7
1991	38.65
1992	38.51
1993	38.64
1994	39.49
1995	40.25
1996	41.61
1997	42.9
1998	44.98
1999	44.86
2000	46.67
2001	51.88
2002	47.39
2003	52.89
2004	53.39
2005	53.58
2006	53.88
2007	54.78
2008	55.14
2009	56.39
2010	57.32
2011	57.64
2012	52.56
2013	53.38
2014	53.64
2016	55.52
2017	56.29