Year	Percent
1970-01-01	63.8
1971-01-01	63.4
1972-01-01	62.9
1973-01-01	63.2
1974-01-01	62.2
1975-01-01	61.2
1976-01-01	61.1
1977-01-01	61.3
1978-01-01	61.3
1979-01-01	61.4
1980-01-01	61.3
1981-01-01	61.2
1982-01-01	61.2
1983-01-01	61.4
1984-01-01	61.0
1985-01-01	60.6
1986-01-01	60.4
1987-01-01	60.1
1988-01-01	60.4
1989-01-01	60.8
1990-01-01	61.3
1991-01-01	61.8
1992-01-01	62.0
1993-01-01	61.7
1994-01-01	61.3
1995-01-01	60.9
1996-01-01	60.9
1997-01-01	61.0
1998-01-01	60.2
1999-01-01	59.4
2000-01-01	59.0
2001-01-01	58.4
2002-01-01	57.5
2003-01-01	57.1
2004-01-01	57.1
2005-01-01	57.3
2006-01-01	57.5
2007-01-01	57.7
2008-01-01	57.4
2009-01-01	56.5
2010-01-01	56.3
2011-01-01	56.2
2012-01-01	56.1