Year	Percent
1971-01-01	16.0
1972-01-01	15.0
1973-01-01	14.1
1974-01-01	13.3
1975-01-01	12.9
1976-01-01	12.3
1977-01-01	11.8
1978-01-01	11.4
1979-01-01	11.1
1980-01-01	10.7
1981-01-01	10.4
1982-01-01	9.6
1983-01-01	9.3
1984-01-01	9.1
1985-01-01	8.7
1986-01-01	8.4
1987-01-01	8.0
1988-01-01	7.7
1989-01-01	7.3
1990-01-01	6.9
1991-01-01	6.6
1992-01-01	6.3
1993-01-01	6.0
1994-01-01	5.7
1995-01-01	5.5
1996-01-01	5.3
1997-01-01	5.2
1998-01-01	5.0
1999-01-01	4.9
2000-01-01	4.6
2001-01-01	4.5
2002-01-01	4.3
2003-01-01	4.2
2004-01-01	3.9
2005-01-01	3.7
2006-01-01	3.8
2007-01-01	3.5
2008-01-01	2.8
2009-01-01	3.0
2010-01-01	3.0
2011-01-01	3.0
2012-01-01	3.0