Year	Percent
1990	19.9
1991	20.8
1992	22.1
1993	23.7
1994	24.7
1995	24.8
1996	25.9
1997	25.7
1998	27.1
1999	28.1
2000	29.5
2001	30.6
2002	31.5
2004	34.4
2005	35.39
2006	35.86
2007	36.64
2008	37.34
2009	38.41