Year	Percent
1971-01-01	83.1
1972-01-01	82.8
1973-01-01	82.3
1974-01-01	81.5
1975-01-01	81.3
1976-01-01	80.9
1977-01-01	80.2
1978-01-01	79.3
1979-01-01	78.8
1980-01-01	78.5
1981-01-01	78.1
1982-01-01	77.0
1983-01-01	76.1
1984-01-01	76.5
1985-01-01	76.1
1986-01-01	75.5
1987-01-01	75.5
1988-01-01	75.7
1989-01-01	75.9
1990-01-01	75.7
1991-01-01	74.7
1992-01-01	73.5
1993-01-01	72.7
1994-01-01	72.3
1995-01-01	72.0
1996-01-01	71.7
1997-01-01	71.5
1998-01-01	71.2
1999-01-01	71.4
2000-01-01	71.1
2001-01-01	70.9
2002-01-01	70.8
2003-01-01	70.8
2004-01-01	70.6
2005-01-01	70.5
2006-01-01	70.8
2007-01-01	70.7
2008-01-01	70.8
2009-01-01	70.2
2010-01-01	69.8
2011-01-01	69.7
2012-01-01	69.8