Year	Percent
1970-01-01	57.8
1971-01-01	58.1
1972-01-01	58.6
1973-01-01	59.7
1974-01-01	60.5
1975-01-01	61.1
1976-01-01	62.5
1977-01-01	62.8
1978-01-01	63.7
1979-01-01	64.5
1980-01-01	65.0
1981-01-01	65.7
1982-01-01	64.9
1983-01-01	65.2
1984-01-01	65.5
1985-01-01	66.0
1986-01-01	66.4
1987-01-01	66.8
1988-01-01	67.1
1989-01-01	67.5
1990-01-01	67.4
1991-01-01	66.8
1992-01-01	65.9
1993-01-01	65.5
1994-01-01	65.2
1995-01-01	64.9
1996-01-01	64.8
1997-01-01	65.1
1998-01-01	65.4
1999-01-01	65.8
2000-01-01	66.0
2001-01-01	66.1
2002-01-01	67.1
2003-01-01	67.7
2004-01-01	67.6
2005-01-01	67.3
2006-01-01	67.2
2007-01-01	67.5
2008-01-01	67.7
2009-01-01	67.2
2010-01-01	67.0
2011-01-01	66.8
2012-01-01	66.7