Year	Percent
1970-01-01	16.9
1971-01-01	15.5
1972-01-01	14.5
1973-01-01	13.1
1974-01-01	12.6
1975-01-01	12.4
1976-01-01	11.9
1977-01-01	11.6
1978-01-01	11.4
1979-01-01	10.8
1980-01-01	10.1
1981-01-01	9.7
1982-01-01	9.4
1983-01-01	8.9
1984-01-01	8.5
1985-01-01	8.4
1986-01-01	8.1
1987-01-01	8.0
1988-01-01	7.6
1989-01-01	7.3
1990-01-01	6.9
1991-01-01	6.4
1992-01-01	6.1
1993-01-01	5.7
1994-01-01	5.6
1995-01-01	5.5
1996-01-01	5.3
1997-01-01	5.1
1998-01-01	5.0
1999-01-01	4.9
2000-01-01	4.8
2001-01-01	4.7
2002-01-01	4.5
2003-01-01	4.4
2004-01-01	4.3
2005-01-01	4.2
2006-01-01	4.1
2007-01-01	4.1
2008-01-01	4.0
2009-01-01	4.0
2010-01-01	3.9
2011-01-01	3.8
2012-01-01	3.7