Year	Percent
1970-01-01	49.0
1971-01-01	48.7
1972-01-01	47.7
1973-01-01	47.6
1974-01-01	47.7
1975-01-01	47.7
1976-01-01	48.0
1977-01-01	48.2
1978-01-01	47.8
1979-01-01	48.0
1980-01-01	48.1
1981-01-01	48.3
1982-01-01	47.7
1983-01-01	47.5
1984-01-01	47.3
1985-01-01	47.2
1986-01-01	47.8
1987-01-01	47.6
1988-01-01	47.4
1989-01-01	47.3
1990-01-01	47.2
1991-01-01	47.8
1992-01-01	47.5
1993-01-01	49.4
1994-01-01	48.8
1995-01-01	48.6
1996-01-01	48.7
1997-01-01	48.7
1998-01-01	49.1
1999-01-01	49.3
2000-01-01	49.5
2001-01-01	49.7
2002-01-01	49.9
2003-01-01	49.6
2004-01-01	49.1
2005-01-01	48.7
2006-01-01	48.9
2007-01-01	48.6
2008-01-01	49.0
2009-01-01	48.4
2010-01-01	48.1
2011-01-01	48.1
2012-01-01	49.0