Year	Percent
1978	24.81
1979	25.37
1980	25.02
1982	29.43
1983	28.9
1984	31.47
1985	32.08
1986	33.31
1987	33.73
1988	33.25
1989	31.78
1990	28.95
1991	28.15
1992	28.32
1993	30.16
1994	31.79
1995	33.47
1996	34.73
1997	34.56
1998	33.76
1999	32.95
2000	31.47
2001	27.83
2002	27.14
2003	26.8
2004	24.44
2005	24.12
2006	21.76
2007	20.36
2008	21.2
2009	25.34
2010	27.9
2011	29.16
2012	35.32
2013	40.04
2014	42.68
2015	40.32
2016	37.77
2017	34.74
2018	32.19
2019	29.16
2020	29.38