Year	Percent
1970-01-01	40.8
1971-01-01	40.4
1972-01-01	41.0
1973-01-01	42.0
1974-01-01	42.6
1975-01-01	42.0
1976-01-01	43.2
1977-01-01	44.5
1978-01-01	46.4
1979-01-01	47.5
1980-01-01	47.7
1981-01-01	48.0
1982-01-01	47.7
1983-01-01	48.0
1984-01-01	49.5
1985-01-01	50.4
1986-01-01	51.4
1987-01-01	52.5
1988-01-01	53.4
1989-01-01	54.3
1990-01-01	54.3
1991-01-01	53.7
1992-01-01	53.8
1993-01-01	54.1
1994-01-01	55.3
1995-01-01	55.6
1996-01-01	56.0
1997-01-01	56.8
1998-01-01	57.1
1999-01-01	57.4
2000-01-01	57.5
2001-01-01	57.0
2002-01-01	56.3
2003-01-01	56.1
2004-01-01	56.0
2005-01-01	56.2
2006-01-01	56.6
2007-01-01	56.6
2008-01-01	56.2
2009-01-01	54.4
2010-01-01	53.6
2011-01-01	53.2
2012-01-01	53.1