Year	Percent
1970-01-01	75.6
1971-01-01	74.7
1972-01-01	74.0
1973-01-01	73.7
1974-01-01	73.5
1975-01-01	72.6
1976-01-01	72.2
1977-01-01	72.0
1978-01-01	71.4
1979-01-01	70.9
1980-01-01	70.2
1981-01-01	69.5
1982-01-01	71.0
1983-01-01	70.2
1984-01-01	69.3
1985-01-01	69.0
1986-01-01	68.6
1987-01-01	67.8
1988-01-01	67.0
1989-01-01	66.6
1990-01-01	65.6
1991-01-01	64.7
1992-01-01	64.5
1993-01-01	63.8
1994-01-01	63.4
1995-01-01	63.1
1996-01-01	63.4
1997-01-01	63.0
1998-01-01	62.4
1999-01-01	62.5
2000-01-01	62.6
2001-01-01	62.1
2002-01-01	62.3
2003-01-01	61.9
2004-01-01	61.5
2005-01-01	61.3
2006-01-01	61.1
2007-01-01	60.9
2008-01-01	61.0
2009-01-01	61.1
2010-01-01	61.0
2011-01-01	60.7
2012-01-01	61.1