Year	Percent
1970-01-01	4.4
1971-01-01	4.3
1972-01-01	4.2
1973-01-01	4.1
1974-01-01	4.0
1975-01-01	4.0
1976-01-01	3.8
1977-01-01	3.6
1978-01-01	3.5
1979-01-01	3.4
1980-01-01	3.4
1981-01-01	3.4
1982-01-01	3.4
1983-01-01	3.4
1984-01-01	3.2
1985-01-01	3.0
1986-01-01	2.9
1987-01-01	2.9
1988-01-01	2.8
1989-01-01	2.7
1990-01-01	2.7
1991-01-01	2.8
1992-01-01	2.7
1993-01-01	2.6
1994-01-01	2.8
1995-01-01	2.8
1996-01-01	2.7
1997-01-01	2.6
1998-01-01	2.6
1999-01-01	2.5
2000-01-01	1.8
2001-01-01	1.7
2002-01-01	1.7
2003-01-01	1.7
2004-01-01	1.6
2005-01-01	1.6
2006-01-01	1.5
2007-01-01	1.4
2008-01-01	1.5
2009-01-01	1.5
2010-01-01	1.6
2011-01-01	1.6
2012-01-01	1.5