Year	Percent
1970-01-01	8.2
1971-01-01	7.8
1972-01-01	7.5
1973-01-01	7.1
1974-01-01	6.7
1975-01-01	6.5
1976-01-01	6.2
1977-01-01	6.1
1978-01-01	6.1
1979-01-01	5.8
1980-01-01	5.6
1981-01-01	5.6
1982-01-01	5.6
1983-01-01	5.5
1984-01-01	5.1
1985-01-01	4.9
1986-01-01	4.7
1987-01-01	4.5
1988-01-01	4.4
1989-01-01	4.3
1990-01-01	4.0
1991-01-01	3.8
1992-01-01	3.8
1993-01-01	4.0
1994-01-01	3.9
1995-01-01	3.5
1996-01-01	3.3
1997-01-01	3.1
1998-01-01	3.0
1999-01-01	3.1
2000-01-01	2.9
2001-01-01	2.6
2002-01-01	2.5
2003-01-01	2.5
2004-01-01	2.5
2005-01-01	2.3
2006-01-01	2.2
2007-01-01	2.2
2008-01-01	2.2
2009-01-01	2.2
2010-01-01	2.1
2011-01-01	2.0
2012-01-01	2.1