Year	Percent
1970-01-01	39.0
1971-01-01	39.4
1972-01-01	39.8
1973-01-01	40.2
1974-01-01	40.6
1975-01-01	40.4
1976-01-01	41.3
1977-01-01	42.0
1978-01-01	42.4
1979-01-01	42.8
1980-01-01	43.1
1981-01-01	43.5
1982-01-01	45.8
1983-01-01	45.8
1984-01-01	46.2
1985-01-01	46.3
1986-01-01	46.8
1987-01-01	46.8
1988-01-01	46.6
1989-01-01	46.9
1990-01-01	46.8
1991-01-01	46.8
1992-01-01	47.2
1993-01-01	47.7
1994-01-01	47.9
1995-01-01	48.1
1996-01-01	48.4
1997-01-01	48.0
1998-01-01	48.4
1999-01-01	48.7
2000-01-01	48.9
2001-01-01	49.0
2002-01-01	49.3
2003-01-01	49.7
2004-01-01	49.8
2005-01-01	49.9
2006-01-01	50.0
2007-01-01	50.3
2008-01-01	50.5
2009-01-01	50.9
2010-01-01	51.0
2011-01-01	50.9
2012-01-01	51.2