Year	Percent
1971-01-01	55.1
1972-01-01	56.1
1973-01-01	56.4
1974-01-01	56.9
1975-01-01	58.5
1976-01-01	59.4
1977-01-01	59.5
1978-01-01	59.8
1979-01-01	60.2
1980-01-01	61.4
1981-01-01	63.2
1982-01-01	64.4
1983-01-01	65.5
1984-01-01	64.3
1985-01-01	65.0
1986-01-01	65.7
1987-01-01	66.6
1988-01-01	66.5
1989-01-01	66.7
1990-01-01	67.3
1991-01-01	66.8
1992-01-01	69.3
1993-01-01	70.0
1994-01-01	71.5
1995-01-01	71.8
1996-01-01	71.8
1997-01-01	74.1
1998-01-01	74.5
1999-01-01	75.4
2000-01-01	76.1
2001-01-01	76.7
2002-01-01	77.3
2003-01-01	78.1
2004-01-01	78.7
2005-01-01	78.8
2006-01-01	78.9
2007-01-01	78.8
2008-01-01	79.5
2009-01-01	80.7
2010-01-01	80.9
2011-01-01	81.1
2012-01-01	81.3