Year	Percent
1970-01-01	7.6
1971-01-01	7.5
1972-01-01	6.9
1973-01-01	6.5
1974-01-01	6.3
1975-01-01	6.1
1976-01-01	5.6
1977-01-01	5.4
1978-01-01	5.2
1979-01-01	5.1
1980-01-01	4.9
1981-01-01	4.8
1982-01-01	4.9
1983-01-01	5.0
1984-01-01	4.8
1985-01-01	4.7
1986-01-01	4.7
1987-01-01	4.5
1988-01-01	4.4
1989-01-01	4.2
1990-01-01	4.2
1991-01-01	4.3
1992-01-01	4.2
1993-01-01	4.3
1994-01-01	4.2
1995-01-01	4.0
1996-01-01	3.9
1997-01-01	3.8
1998-01-01	3.8
1999-01-01	3.5
2000-01-01	3.3
2001-01-01	2.8
2002-01-01	2.8
2003-01-01	2.7
2004-01-01	2.7
2005-01-01	2.7
2006-01-01	2.6
2007-01-01	2.5
2008-01-01	2.3
2009-01-01	2.3
2010-01-01	2.2
2011-01-01	2.1
2012-01-01	2.2