Year	Percent
1970-01-01	74.4
1971-01-01	73.5
1972-01-01	72.6
1973-01-01	72.5
1974-01-01	72.2
1975-01-01	70.6
1976-01-01	70.2
1977-01-01	69.7
1978-01-01	69.0
1979-01-01	68.2
1980-01-01	67.5
1981-01-01	66.1
1982-01-01	67.2
1983-01-01	66.1
1984-01-01	64.4
1985-01-01	63.8
1986-01-01	63.3
1987-01-01	62.7
1988-01-01	62.2
1989-01-01	62.3
1990-01-01	61.5
1991-01-01	60.5
1992-01-01	59.8
1993-01-01	58.1
1994-01-01	57.4
1995-01-01	57.6
1996-01-01	57.5
1997-01-01	56.9
1998-01-01	56.7
1999-01-01	57.0
2000-01-01	58.0
2001-01-01	58.0
2002-01-01	57.8
2003-01-01	57.1
2004-01-01	56.5
2005-01-01	56.2
2006-01-01	56.1
2007-01-01	56.3
2008-01-01	56.7
2009-01-01	55.6
2010-01-01	55.4
2011-01-01	55.3
2012-01-01	55.1