Year	Percent
1981	2.77
1982	4.07
1983	6.06
1984	6.37
1985	6
1986	5.56
1987	5.48
1988	5.72
1989	5.47
1990	5.15
1991	5.74
1992	6.01
1993	6.65
1994	6.99
1995	7.3
1996	7.7
1997	7.69
1998	8.96
1999	9.9
2000	9.67
2001	8.94
2002	8.76
2003	8.36
2004	9.16
2005	9.18
2006	8.19
2007	7.82
2008	7.23
2009	9.04
2010	12.11
2011	17.24
2012	23.72
2013	26.88
2014	26.02
2015	24.44
2016	22.82
2017	20.73
2018	18.88
2019	17.12
2020	16.19