Year	Percent
1970-01-01	3.3
1971-01-01	4.0
1972-01-01	3.6
1973-01-01	3.1
1974-01-01	3.6
1975-01-01	6.0
1976-01-01	5.5
1977-01-01	4.9
1978-01-01	4.1
1979-01-01	3.9
1980-01-01	5.1
1981-01-01	5.4
1982-01-01	7.4
1983-01-01	7.5
1984-01-01	5.8
1985-01-01	5.6
1986-01-01	5.4
1987-01-01	4.8
1988-01-01	4.3
1989-01-01	4.0
1990-01-01	4.4
1991-01-01	5.4
1992-01-01	6.1
1993-01-01	5.6
1994-01-01	4.8
1995-01-01	4.3
1996-01-01	4.2
1997-01-01	3.8
1998-01-01	3.4
1999-01-01	3.1
2000-01-01	3.0
2001-01-01	3.7
2002-01-01	4.6
2003-01-01	4.8
2004-01-01	4.4
2005-01-01	4.0
2006-01-01	3.6
2007-01-01	3.6
2008-01-01	4.6
2009-01-01	7.9
2010-01-01	8.2
2011-01-01	7.6
2012-01-01	6.8