Year	Percent
1970-01-01	39.3
1971-01-01	39.7
1972-01-01	39.6
1973-01-01	40.9
1974-01-01	41.7
1975-01-01	41.4
1976-01-01	41.5
1977-01-01	41.5
1978-01-01	41.0
1979-01-01	40.7
1980-01-01	41.9
1981-01-01	42.1
1982-01-01	41.5
1983-01-01	40.8
1984-01-01	41.8
1985-01-01	43.0
1986-01-01	45.0
1987-01-01	45.6
1988-01-01	46.8
1989-01-01	48.7
1990-01-01	49.4
1991-01-01	48.2
1992-01-01	47.8
1993-01-01	47.6
1994-01-01	48.6
1995-01-01	50.3
1996-01-01	50.4
1997-01-01	50.4
1998-01-01	51.0
1999-01-01	51.3
2000-01-01	52.5
2001-01-01	52.6
2002-01-01	52.9
2003-01-01	53.6
2004-01-01	53.7
2005-01-01	55.0
2006-01-01	55.7
2007-01-01	56.4
2008-01-01	57.2
2009-01-01	56.9
2010-01-01	56.6
2011-01-01	56.8
2012-01-01	56.7