Year	Percent
1970-01-01	81.5
1971-01-01	81.9
1972-01-01	81.8
1973-01-01	81.8
1974-01-01	81.6
1975-01-01	81.0
1976-01-01	81.0
1977-01-01	80.3
1978-01-01	80.0
1979-01-01	79.7
1980-01-01	79.3
1981-01-01	79.3
1982-01-01	79.0
1983-01-01	79.1
1984-01-01	78.4
1985-01-01	77.6
1986-01-01	77.3
1987-01-01	76.6
1988-01-01	76.5
1989-01-01	76.4
1990-01-01	76.7
1991-01-01	77.1
1992-01-01	77.5
1993-01-01	77.6
1994-01-01	77.2
1995-01-01	77.0
1996-01-01	77.1
1997-01-01	77.1
1998-01-01	76.6
1999-01-01	76.1
2000-01-01	75.5
2001-01-01	74.8
2002-01-01	73.8
2003-01-01	73.0
2004-01-01	72.4
2005-01-01	72.3
2006-01-01	72.2
2007-01-01	72.4
2008-01-01	72.0
2009-01-01	71.3
2010-01-01	70.9
2011-01-01	70.5
2012-01-01	69.8