Year	Percent
1970-01-01	49.2
1971-01-01	48.6
1972-01-01	47.8
1973-01-01	48.0
1974-01-01	46.5
1975-01-01	45.8
1976-01-01	45.9
1977-01-01	46.9
1978-01-01	47.5
1979-01-01	47.6
1980-01-01	47.2
1981-01-01	47.3
1982-01-01	47.6
1983-01-01	48.5
1984-01-01	48.0
1985-01-01	47.8
1986-01-01	47.8
1987-01-01	47.8
1988-01-01	48.0
1989-01-01	48.7
1990-01-01	49.3
1991-01-01	49.8
1992-01-01	50.0
1993-01-01	49.7
1994-01-01	49.5
1995-01-01	49.3
1996-01-01	49.4
1997-01-01	49.7
1998-01-01	49.4
1999-01-01	48.8
2000-01-01	48.7
2001-01-01	48.5
2002-01-01	47.9
2003-01-01	47.6
2004-01-01	47.6
2005-01-01	47.6
2006-01-01	47.9
2007-01-01	48.1
2008-01-01	48.0
2009-01-01	48.1
2010-01-01	48.1
2011-01-01	47.7
2012-01-01	47.7