Year	Percent
1970-01-01	39.6
1971-01-01	40.3
1972-01-01	40.8
1973-01-01	41.1
1974-01-01	41.9
1975-01-01	42.7
1976-01-01	43.1
1977-01-01	43.8
1978-01-01	44.4
1979-01-01	44.8
1980-01-01	45.3
1981-01-01	46.1
1982-01-01	46.4
1983-01-01	46.7
1984-01-01	47.0
1985-01-01	47.2
1986-01-01	47.3
1987-01-01	47.7
1988-01-01	47.8
1989-01-01	47.8
1990-01-01	47.8
1991-01-01	47.7
1992-01-01	47.7
1993-01-01	47.7
1994-01-01	47.6
1995-01-01	47.6
1996-01-01	47.7
1997-01-01	47.5
1998-01-01	47.2
1999-01-01	47.4
2000-01-01	47.5
2001-01-01	47.7
2002-01-01	47.8
2003-01-01	47.8
2004-01-01	47.8
2005-01-01	47.5
2006-01-01	47.5
2007-01-01	47.6
2008-01-01	47.5
2009-01-01	47.4
2010-01-01	47.1
2011-01-01	47.3
2012-01-01	47.5