Year	Percent
1970-01-01	43.3
1971-01-01	43.4
1972-01-01	43.9
1973-01-01	44.7
1974-01-01	45.7
1975-01-01	46.3
1976-01-01	47.3
1977-01-01	48.4
1978-01-01	50.0
1979-01-01	50.9
1980-01-01	51.5
1981-01-01	52.1
1982-01-01	52.6
1983-01-01	52.9
1984-01-01	53.6
1985-01-01	54.5
1986-01-01	55.3
1987-01-01	56.0
1988-01-01	56.6
1989-01-01	57.4
1990-01-01	57.5
1991-01-01	57.4
1992-01-01	57.8
1993-01-01	57.9
1994-01-01	58.8
1995-01-01	58.9
1996-01-01	59.3
1997-01-01	59.8
1998-01-01	59.8
1999-01-01	60.0
2000-01-01	59.9
2001-01-01	59.8
2002-01-01	59.6
2003-01-01	59.5
2004-01-01	59.2
2005-01-01	59.3
2006-01-01	59.4
2007-01-01	59.3
2008-01-01	59.5
2009-01-01	59.2
2010-01-01	58.6
2011-01-01	58.1
2012-01-01	57.7