Year	Percent
1970-01-01	56.6
1971-01-01	56.2
1972-01-01	55.8
1973-01-01	55.9
1974-01-01	54.8
1975-01-01	53.2
1976-01-01	52.8
1977-01-01	52.5
1978-01-01	52.6
1979-01-01	52.9
1980-01-01	53.1
1981-01-01	52.5
1982-01-01	51.6
1983-01-01	50.6
1984-01-01	50.6
1985-01-01	50.8
1986-01-01	51.3
1987-01-01	51.5
1988-01-01	51.6
1989-01-01	52.0
1990-01-01	52.3
1991-01-01	55.5
1992-01-01	54.2
1993-01-01	53.2
1994-01-01	52.6
1995-01-01	52.4
1996-01-01	52.0
1997-01-01	51.6
1998-01-01	52.3
1999-01-01	52.1
2000-01-01	52.2
2001-01-01	52.2
2002-01-01	51.5
2003-01-01	50.8
2004-01-01	50.6
2005-01-01	51.1
2006-01-01	52.1
2007-01-01	53.2
2008-01-01	54.0
2009-01-01	54.0
2010-01-01	54.4
2011-01-01	55.7
2012-01-01	55.9