Year	Percent
1970-01-01	2.2
1971-01-01	3.8
1972-01-01	4.6
1973-01-01	4.5
1974-01-01	3.3
1975-01-01	2.8
1976-01-01	2.8
1977-01-01	3.2
1978-01-01	4.3
1979-01-01	3.7
1980-01-01	3.7
1981-01-01	4.8
1982-01-01	6.1
1983-01-01	6.9
1984-01-01	6.7
1985-01-01	6.4
1986-01-01	6.3
1987-01-01	5.1
1988-01-01	3.9
1989-01-01	3.3
1990-01-01	3.4
1991-01-01	6.7
1992-01-01	13.2
1993-01-01	22.6
1994-01-01	23.5
1995-01-01	19.9
1996-01-01	21.1
1997-01-01	20.2
1998-01-01	15.8
1999-01-01	13.2
2000-01-01	9.6
2001-01-01	9.9
2002-01-01	10.7
2003-01-01	11.9
2004-01-01	14.8
2005-01-01	17.6
2006-01-01	16.7
2007-01-01	13.7
2008-01-01	14.1
2009-01-01	20.0
2010-01-01	19.9
2011-01-01	18.1
2012-01-01	19.0