Year	Percent
1970-01-01	35.7
1971-01-01	35.9
1972-01-01	36.2
1973-01-01	37.0
1974-01-01	36.8
1975-01-01	35.6
1976-01-01	35.6
1977-01-01	35.1
1978-01-01	34.8
1979-01-01	34.7
1980-01-01	35.1
1981-01-01	35.1
1982-01-01	34.5
1983-01-01	34.4
1984-01-01	34.5
1985-01-01	34.6
1986-01-01	34.2
1987-01-01	33.5
1988-01-01	33.9
1989-01-01	34.1
1990-01-01	33.9
1991-01-01	34.2
1992-01-01	34.3
1993-01-01	34.0
1994-01-01	33.7
1995-01-01	33.1
1996-01-01	32.9
1997-01-01	32.8
1998-01-01	31.7
1999-01-01	31.3
2000-01-01	30.9
2001-01-01	30.1
2002-01-01	29.0
2003-01-01	28.5
2004-01-01	27.6
2005-01-01	27.1
2006-01-01	27.2
2007-01-01	27.0
2008-01-01	26.6
2009-01-01	25.6
2010-01-01	25.0
2011-01-01	24.9
2012-01-01	24.7