Year	Percent
1977	12.48
1978	17.56
1979	21.56
1980	28.39
1981	34.69
1982	38.61
1983	41.86
1984	46.27
1985	47.9
1986	46.46
1987	43.46
1988	41.12
1989	34.31
1990	31.84
1991	30.51
1992	33.08
1993	42.25
1994	44.95
1995	41.85
1996	41.82
1997	39.1
1998	35.44
1999	29.13
2000	25.25
2001	20.7
2002	21.5
2003	22.28
2004	22.46
2005	19.63
2006	17.89
2007	18.09
2008	24.45
2009	37.73
2010	41.48
2011	46.19
2012	52.86
2013	55.48
2014	53.2
2015	48.33
2016	44.44
2017	38.57
2018	34.34
2019	32.53
2020	38.26