Year	Percent
1970-01-01	38.3
1971-01-01	39.4
1972-01-01	40.2
1973-01-01	41.9
1974-01-01	43.0
1975-01-01	44.4
1976-01-01	46.2
1977-01-01	47.1
1978-01-01	48.5
1979-01-01	49.8
1980-01-01	51.0
1981-01-01	52.3
1982-01-01	52.3
1983-01-01	53.1
1984-01-01	53.9
1985-01-01	55.0
1986-01-01	55.7
1987-01-01	56.5
1988-01-01	57.4
1989-01-01	58.1
1990-01-01	58.5
1991-01-01	58.4
1992-01-01	57.8
1993-01-01	57.6
1994-01-01	57.4
1995-01-01	57.3
1996-01-01	57.3
1997-01-01	57.8
1998-01-01	58.5
1999-01-01	59.1
2000-01-01	59.5
2001-01-01	59.8
2002-01-01	60.9
2003-01-01	61.9
2004-01-01	61.9
2005-01-01	61.6
2006-01-01	61.8
2007-01-01	62.5
2008-01-01	62.6
2009-01-01	62.5
2010-01-01	62.4
2011-01-01	62.2
2012-01-01	62.1