Year	Percent
1975-01-01	0.3
1976-01-01	0.7
1977-01-01	0.4
1978-01-01	0.3
1979-01-01	0.3
1980-01-01	0.2
1981-01-01	0.2
1982-01-01	0.4
1983-01-01	0.9
1984-01-01	1.1
1985-01-01	0.9
1986-01-01	0.7
1987-01-01	0.7
1988-01-01	0.6
1989-01-01	0.5
1990-01-01	0.5
1991-01-01	1.7
1992-01-01	2.7
1993-01-01	3.6
1994-01-01	3.6
1995-01-01	3.2
1996-01-01	3.5
1997-01-01	3.9
1998-01-01	3.3
1999-01-01	2.8
2000-01-01	2.5
2001-01-01	2.2
2002-01-01	2.9
2003-01-01	3.9
2004-01-01	4.1
2005-01-01	4.2
2006-01-01	3.8
2007-01-01	3.4
2008-01-01	3.2
2009-01-01	4.1
2010-01-01	4.2
2011-01-01	3.8