Year	Percent
1981	38.89
1982	38.01
1983	37.82
1984	38.18
1985	38.09
1986	37.47
1987	37.53
1988	37.71
1989	38
1990	37.03
1991	35.98
1992	34.59
1994	31.94
1995	31.97
1996	31.67
2000	31.06
2001	30.73
2002	28.54
2003	28.55
2004	29.04
2005	29.24
2006	29.97
2007	30.72
2008	31.87
2009	31.1
2010	30.28
2011	30.66
2012	30.4
2013	30.52
2014	30.49
2015	30.44
2016	31.33
2017	31.54