Year	Percent
1970-01-01	48.1
1971-01-01	47.2
1972-01-01	46.2
1973-01-01	46.7
1974-01-01	45.1
1975-01-01	44.0
1976-01-01	44.1
1977-01-01	44.9
1978-01-01	45.5
1979-01-01	45.6
1980-01-01	45.6
1981-01-01	45.6
1982-01-01	45.9
1983-01-01	46.7
1984-01-01	46.4
1985-01-01	46.3
1986-01-01	46.2
1987-01-01	46.2
1988-01-01	46.6
1989-01-01	47.4
1990-01-01	48.0
1991-01-01	48.6
1992-01-01	48.7
1993-01-01	48.2
1994-01-01	48.0
1995-01-01	47.7
1996-01-01	47.6
1997-01-01	47.9
1998-01-01	47.4
1999-01-01	46.6
2000-01-01	46.4
2001-01-01	46.1
2002-01-01	45.4
2003-01-01	45.3
2004-01-01	45.5
2005-01-01	45.7
2006-01-01	46.0
2007-01-01	46.1
2008-01-01	45.9
2009-01-01	45.6
2010-01-01	45.7
2011-01-01	45.6
2012-01-01	45.7