Year	Percent
1981	24.69
1982	23.4
1983	25.36
1984	21.76
1985	22.1
1986	24.16
1987	23.82
1988	23.34
1989	23.67
1990	22.69
1992	20.35
1993	20.69
1995	22.13
1996	22.21
1997	22.24
1998	22.14
1999	21.41
2001	20.93
2002	21.43
2003	20.91
2004	21.03
2005	21.38
2006	21.34
2007	21.91
2008	22.66
2009	22.12
2011	21.75
2012	23.01
2013	22.93
2014	22.88
2015	22.16
2016	20.86
2017	20.45