Year	Percent
1976	23.57
1977	27.68
1978	29.81
1979	22.24
1980	30.44
1982	30.43
1985	31.82
1986	36.52
1987	36.73
1988	30.41
1989	30.44
1990	30.23
1991	31.36
1992	30.92
1993	33.55
1994	35.01
1995	37.6
1996	37.88
1997	39.81
1998	38.76
1999	38.95
2000	37.28
2001	37.49
2002	36.9
2003	35.91
2004	38.66
2005	37.22
2006	39.33
2007	39.9
2008	40.43
2009	41.23
2010	42.2
2011	42.82
2012	43
2013	44.07
2014	44.32
2015	44.92
2016	46.46
2017	47.19