Year	Percent
1970-01-01	78.4
1971-01-01	77.1
1972-01-01	75.9
1973-01-01	75.3
1974-01-01	73.5
1975-01-01	70.9
1976-01-01	70.3
1977-01-01	69.9
1978-01-01	69.7
1979-01-01	69.8
1980-01-01	69.6
1981-01-01	68.5
1982-01-01	67.0
1983-01-01	65.8
1984-01-01	65.7
1985-01-01	65.8
1986-01-01	66.3
1987-01-01	66.2
1988-01-01	65.9
1989-01-01	65.9
1990-01-01	65.6
1991-01-01	67.9
1992-01-01	66.3
1993-01-01	64.7
1994-01-01	63.8
1995-01-01	63.1
1996-01-01	62.1
1997-01-01	61.3
1998-01-01	61.9
1999-01-01	60.7
2000-01-01	60.6
2001-01-01	60.2
2002-01-01	58.9
2003-01-01	57.5
2004-01-01	57.3
2005-01-01	57.7
2006-01-01	58.6
2007-01-01	59.7
2008-01-01	60.5
2009-01-01	59.9
2010-01-01	60.2
2011-01-01	61.5
2012-01-01	61.7