Year	Percent
1981	19
1982	18
1983	16
1984	22
1985	27
1986	27
1987	27
1988	24
1989	14
1990	25
1991	28
1992	21
1993	19
1994	19
1995	40
1996	38
1997	35
1998	35
1999	35
2000	37
2001	39
2002	41
2003	43
2004	45
2005	47
2006	53
2007	60
2008	66
2009	69
2010	71
2011	76