Year	Percent
1970-01-01	42.8
1971-01-01	44.4
1972-01-01	45.4
1973-01-01	46.0
1974-01-01	47.2
1975-01-01	48.8
1976-01-01	49.6
1977-01-01	50.3
1978-01-01	50.8
1979-01-01	51.4
1980-01-01	51.9
1981-01-01	52.8
1982-01-01	53.7
1983-01-01	54.5
1984-01-01	55.1
1985-01-01	55.4
1986-01-01	55.8
1987-01-01	56.4
1988-01-01	57.0
1989-01-01	57.4
1990-01-01	57.6
1991-01-01	56.1
1992-01-01	57.3
1993-01-01	58.7
1994-01-01	60.0
1995-01-01	61.5
1996-01-01	62.6
1997-01-01	63.2
1998-01-01	63.6
1999-01-01	64.0
2000-01-01	64.5
2001-01-01	65.1
2002-01-01	65.8
2003-01-01	66.4
2004-01-01	66.9
2005-01-01	68.5
2006-01-01	68.7
2007-01-01	68.5
2008-01-01	68.7
2009-01-01	70.8
2010-01-01	71.3
2011-01-01	71.5
2012-01-01	71.6