Year	Percent
1970-01-01	37.3
1971-01-01	37.6
1972-01-01	37.9
1973-01-01	38.4
1974-01-01	38.6
1975-01-01	38.4
1976-01-01	38.9
1977-01-01	39.3
1978-01-01	39.6
1979-01-01	39.7
1980-01-01	39.6
1981-01-01	39.5
1982-01-01	41.6
1983-01-01	41.5
1984-01-01	41.4
1985-01-01	41.3
1986-01-01	41.7
1987-01-01	41.5
1988-01-01	41.4
1989-01-01	41.8
1990-01-01	42.0
1991-01-01	42.0
1992-01-01	41.9
1993-01-01	42.1
1994-01-01	41.9
1995-01-01	42.4
1996-01-01	42.5
1997-01-01	42.2
1998-01-01	42.6
1999-01-01	43.1
2000-01-01	44.0
2001-01-01	44.5
2002-01-01	44.9
2003-01-01	44.9
2004-01-01	44.9
2005-01-01	45.0
2006-01-01	45.1
2007-01-01	46.0
2008-01-01	46.5
2009-01-01	46.2
2010-01-01	46.0
2011-01-01	46.0
2012-01-01	46.0