Year	Percent
1970-01-01	1.6
1971-01-01	1.6
1972-01-01	1.6
1973-01-01	1.7
1974-01-01	1.3
1975-01-01	1.5
1976-01-01	1.7
1977-01-01	2.0
1978-01-01	1.9
1979-01-01	2.1
1980-01-01	2.0
1981-01-01	2.3
1982-01-01	2.5
1983-01-01	2.8
1984-01-01	2.8
1985-01-01	2.9
1986-01-01	3.9
1987-01-01	4.3
1988-01-01	4.5
1989-01-01	4.7
1990-01-01	4.2
1991-01-01	4.2
1992-01-01	4.6
1993-01-01	6.6
1994-01-01	7.6
1995-01-01	8.2
1996-01-01	8.3
1997-01-01	8.6
1998-01-01	8.8
1999-01-01	8.6
2000-01-01	8.0
2001-01-01	7.4
2002-01-01	7.0
2003-01-01	6.9
2004-01-01	6.6
2005-01-01	6.3
2006-01-01	5.6
2007-01-01	5.0
2008-01-01	5.6
2009-01-01	6.5
2010-01-01	7.1
2011-01-01	7.0
2012-01-01	9.0