Year	Percent
1971-01-01	44.6
1972-01-01	44.9
1973-01-01	45.8
1974-01-01	46.4
1975-01-01	46.6
1976-01-01	46.7
1977-01-01	46.9
1978-01-01	47.4
1979-01-01	48.0
1980-01-01	48.6
1981-01-01	48.6
1982-01-01	48.1
1983-01-01	48.4
1984-01-01	50.2
1985-01-01	50.7
1986-01-01	51.4
1987-01-01	52.1
1988-01-01	52.8
1989-01-01	53.6
1990-01-01	53.9
1991-01-01	53.6
1992-01-01	53.2
1993-01-01	53.3
1994-01-01	53.3
1995-01-01	53.5
1996-01-01	53.8
1997-01-01	54.2
1998-01-01	54.4
1999-01-01	54.9
2000-01-01	55.2
2001-01-01	55.1
2002-01-01	55.5
2003-01-01	55.6
2004-01-01	55.8
2005-01-01	56.1
2006-01-01	56.6
2007-01-01	56.4
2008-01-01	56.7
2009-01-01	56.8
2010-01-01	56.8
2011-01-01	57.0
2012-01-01	57.2