Year	Percent
1979	0.01
1990	10.1
1991	13.6
1992	17.2
1993	20.6
1994	23.3
1995	25.1
1996	26.2
1997	26.5
1998	26.3
1999	25.7
2000	24.8
2001	23.7
2002	22.5
2003	21.1
2004	19.8
2005	18.4
2006	17.2
2007	16.1
2008	15.1
2009	15.7
2010	15.2
2011	14.9