Year	Percent
1970-01-01	8.5
1971-01-01	8.0
1972-01-01	7.6
1973-01-01	7.2
1974-01-01	6.9
1975-01-01	6.7
1976-01-01	6.2
1977-01-01	5.9
1978-01-01	5.7
1979-01-01	5.3
1980-01-01	5.2
1981-01-01	5.0
1982-01-01	4.9
1983-01-01	4.8
1984-01-01	4.7
1985-01-01	4.5
1986-01-01	4.3
1987-01-01	4.1
1988-01-01	3.9
1989-01-01	3.7
1990-01-01	3.5
1991-01-01	4.1
1992-01-01	3.8
1993-01-01	3.5
1994-01-01	3.3
1995-01-01	3.1
1996-01-01	3.0
1997-01-01	2.9
1998-01-01	2.8
1999-01-01	2.8
2000-01-01	2.6
2001-01-01	2.6
2002-01-01	2.5
2003-01-01	2.5
2004-01-01	2.4
2005-01-01	2.4
2006-01-01	2.3
2007-01-01	2.3
2008-01-01	2.3
2009-01-01	1.7
2010-01-01	1.6
2011-01-01	1.6
2012-01-01	1.6