Year	Percent
1970-01-01	24.4
1971-01-01	24.1
1972-01-01	23.5
1973-01-01	23.3
1974-01-01	22.9
1975-01-01	21.3
1976-01-01	21.2
1977-01-01	20.9
1978-01-01	19.9
1979-01-01	19.6
1980-01-01	19.4
1981-01-01	19.2
1982-01-01	18.6
1983-01-01	17.8
1984-01-01	17.3
1985-01-01	16.1
1986-01-01	15.6
1987-01-01	15.4
1988-01-01	15.3
1989-01-01	15.1
1990-01-01	14.4
1991-01-01	13.8
1992-01-01	13.7
1993-01-01	13.5
1994-01-01	13.5
1995-01-01	13.0
1996-01-01	12.8
1997-01-01	12.9
1998-01-01	12.3
1999-01-01	11.8
2000-01-01	12.1
2001-01-01	11.6
2002-01-01	11.5
2003-01-01	11.0
2004-01-01	10.9
2005-01-01	10.4
2006-01-01	10.0
2007-01-01	9.9
2008-01-01	9.8
2009-01-01	9.3
2010-01-01	8.9
2011-01-01	8.5
2012-01-01	8.4