Year	Percent
1970-01-01	36.1
1971-01-01	36.8
1972-01-01	37.4
1973-01-01	39.1
1974-01-01	40.2
1975-01-01	40.7
1976-01-01	42.6
1977-01-01	42.9
1978-01-01	44.0
1979-01-01	45.7
1980-01-01	46.9
1981-01-01	48.2
1982-01-01	47.0
1983-01-01	47.3
1984-01-01	48.2
1985-01-01	49.5
1986-01-01	50.7
1987-01-01	51.6
1988-01-01	53.0
1989-01-01	53.9
1990-01-01	54.1
1991-01-01	53.2
1992-01-01	52.4
1993-01-01	51.9
1994-01-01	52.4
1995-01-01	52.7
1996-01-01	52.6
1997-01-01	53.2
1998-01-01	54.3
1999-01-01	55.2
2000-01-01	56.0
2001-01-01	56.2
2002-01-01	57.1
2003-01-01	58.0
2004-01-01	58.2
2005-01-01	58.1
2006-01-01	58.6
2007-01-01	59.5
2008-01-01	59.6
2009-01-01	58.7
2010-01-01	58.5
2011-01-01	58.5
2012-01-01	58.5