Year	Percent
1970-01-01	56.9
1971-01-01	56.5
1972-01-01	56.2
1973-01-01	56.3
1974-01-01	55.7
1975-01-01	55.0
1976-01-01	54.6
1977-01-01	54.4
1978-01-01	54.4
1979-01-01	54.5
1980-01-01	54.7
1981-01-01	54.7
1982-01-01	54.6
1983-01-01	54.3
1984-01-01	54.4
1985-01-01	54.7
1986-01-01	54.9
1987-01-01	55.0
1988-01-01	55.1
1989-01-01	55.2
1990-01-01	55.0
1991-01-01	58.8
1992-01-01	58.1
1993-01-01	57.8
1994-01-01	57.4
1995-01-01	57.1
1996-01-01	57.1
1997-01-01	57.3
1998-01-01	57.7
1999-01-01	56.9
2000-01-01	56.7
2001-01-01	56.7
2002-01-01	56.4
2003-01-01	56.0
2004-01-01	56.4
2005-01-01	57.5
2006-01-01	58.1
2007-01-01	58.3
2008-01-01	58.4
2009-01-01	58.5
2010-01-01	58.6
2011-01-01	59.2
2012-01-01	59.2