Year	Percent
1970-01-01	56.2
1971-01-01	56.1
1972-01-01	55.9
1973-01-01	56.1
1974-01-01	56.2
1975-01-01	55.7
1976-01-01	55.9
1977-01-01	56.2
1978-01-01	56.1
1979-01-01	56.1
1980-01-01	56.0
1981-01-01	55.8
1982-01-01	57.7
1983-01-01	57.3
1984-01-01	57.1
1985-01-01	57.0
1986-01-01	57.1
1987-01-01	56.7
1988-01-01	56.2
1989-01-01	56.2
1990-01-01	55.7
1991-01-01	55.3
1992-01-01	55.4
1993-01-01	55.3
1994-01-01	55.2
1995-01-01	55.2
1996-01-01	55.5
1997-01-01	55.1
1998-01-01	55.1
1999-01-01	55.2
2000-01-01	55.4
2001-01-01	55.2
2002-01-01	55.4
2003-01-01	55.5
2004-01-01	55.4
2005-01-01	55.3
2006-01-01	55.3
2007-01-01	55.4
2008-01-01	55.5
2009-01-01	55.8
2010-01-01	55.8
2011-01-01	55.6
2012-01-01	55.9