Year	Percent
1970-01-01	71.9
1971-01-01	71.6
1972-01-01	69.9
1973-01-01	69.3
1974-01-01	69.4
1975-01-01	68.8
1976-01-01	68.5
1977-01-01	67.5
1978-01-01	66.8
1979-01-01	66.3
1980-01-01	66.0
1981-01-01	65.6
1982-01-01	64.4
1983-01-01	63.5
1984-01-01	62.9
1985-01-01	62.5
1986-01-01	61.8
1987-01-01	60.9
1988-01-01	60.4
1989-01-01	59.9
1990-01-01	60.0
1991-01-01	60.6
1992-01-01	59.8
1993-01-01	59.6
1994-01-01	58.1
1995-01-01	57.3
1996-01-01	57.1
1997-01-01	56.9
1998-01-01	56.9
1999-01-01	57.1
2000-01-01	57.4
2001-01-01	57.7
2002-01-01	58.1
2003-01-01	57.6
2004-01-01	56.9
2005-01-01	56.8
2006-01-01	57.3
2007-01-01	57.3
2008-01-01	56.9
2009-01-01	55.3
2010-01-01	54.4
2011-01-01	54.2
2012-01-01	53.2