Year	Percent
1981	45.9
1982	46.2
1983	46.8
1984	46.5
1985	42.5
1986	46.6
1987	47.4
1988	49.2
1989	51.1
1990	48.1
1992	39.7
1993	48.4
1995	47.5
1996	47.4
1997	47.4
1998	47.6
1999	47
2001	46.1
2002	46.1
2003	45.4
2004	44.5
2005	44.6
2006	44.1
2007	43.2
2008	42.5
2009	42.2
2011	41.7
2012	41.6
2013	41.7
2014	40.6
2015	40.4