Year	Percent
1970-01-01	36.6
1971-01-01	37.0
1972-01-01	37.4
1973-01-01	37.7
1974-01-01	38.0
1975-01-01	38.2
1976-01-01	38.8
1977-01-01	39.4
1978-01-01	39.8
1979-01-01	40.2
1980-01-01	40.6
1981-01-01	41.0
1982-01-01	41.8
1983-01-01	42.1
1984-01-01	42.6
1985-01-01	42.8
1986-01-01	43.2
1987-01-01	43.5
1988-01-01	43.7
1989-01-01	43.9
1990-01-01	44.2
1991-01-01	44.5
1992-01-01	44.8
1993-01-01	45.3
1994-01-01	45.5
1995-01-01	45.8
1996-01-01	45.9
1997-01-01	45.8
1998-01-01	46.2
1999-01-01	46.3
2000-01-01	46.4
2001-01-01	46.6
2002-01-01	46.7
2003-01-01	47.0
2004-01-01	47.3
2005-01-01	47.4
2006-01-01	47.5
2007-01-01	47.8
2008-01-01	47.8
2009-01-01	48.0
2010-01-01	48.0
2011-01-01	48.1
2012-01-01	48.0