Year	Percent
1970-01-01	77.8
1971-01-01	77.3
1972-01-01	77.5
1973-01-01	78.2
1974-01-01	78.7
1975-01-01	78.4
1976-01-01	79.2
1977-01-01	79.0
1978-01-01	79.3
1979-01-01	79.6
1980-01-01	79.5
1981-01-01	79.4
1982-01-01	77.9
1983-01-01	77.7
1984-01-01	77.5
1985-01-01	77.5
1986-01-01	77.6
1987-01-01	77.4
1988-01-01	77.2
1989-01-01	77.2
1990-01-01	76.6
1991-01-01	75.5
1992-01-01	74.4
1993-01-01	73.7
1994-01-01	73.4
1995-01-01	72.8
1996-01-01	72.5
1997-01-01	72.7
1998-01-01	72.7
1999-01-01	72.9
2000-01-01	72.8
2001-01-01	72.7
2002-01-01	73.4
2003-01-01	73.7
2004-01-01	73.5
2005-01-01	73.2
2006-01-01	72.6
2007-01-01	72.8
2008-01-01	73.0
2009-01-01	72.0
2010-01-01	71.8
2011-01-01	71.7
2012-01-01	71.4