Year	Percent
1970-01-01	8.1
1971-01-01	7.7
1972-01-01	7.9
1973-01-01	7.4
1974-01-01	7.0
1975-01-01	6.9
1976-01-01	6.6
1977-01-01	6.7
1978-01-01	6.4
1979-01-01	6.5
1980-01-01	6.5
1981-01-01	6.5
1982-01-01	6.5
1983-01-01	6.6
1984-01-01	6.2
1985-01-01	6.1
1986-01-01	6.1
1987-01-01	5.8
1988-01-01	5.8
1989-01-01	5.5
1990-01-01	5.6
1991-01-01	5.5
1992-01-01	5.3
1993-01-01	5.3
1994-01-01	5.1
1995-01-01	4.9
1996-01-01	5.0
1997-01-01	5.1
1998-01-01	4.9
1999-01-01	4.9
2000-01-01	4.9
2001-01-01	4.8
2002-01-01	4.4
2003-01-01	3.9
2004-01-01	3.7
2005-01-01	3.6
2006-01-01	3.4
2007-01-01	3.3
2008-01-01	3.3
2009-01-01	3.3
2010-01-01	3.3
2011-01-01	2.9
2012-01-01	2.9