Year	Percent
1970-01-01	1.7
1971-01-01	2.8
1972-01-01	3.0
1973-01-01	2.8
1974-01-01	2.4
1975-01-01	2.0
1976-01-01	2.0
1977-01-01	2.2
1978-01-01	2.4
1979-01-01	2.3
1980-01-01	2.3
1981-01-01	2.7
1982-01-01	3.4
1983-01-01	3.5
1984-01-01	3.2
1985-01-01	2.9
1986-01-01	2.7
1987-01-01	2.3
1988-01-01	1.9
1989-01-01	1.7
1990-01-01	1.8
1991-01-01	2.9
1992-01-01	4.6
1993-01-01	7.6
1994-01-01	8.2
1995-01-01	8.1
1996-01-01	9.3
1997-01-01	9.7
1998-01-01	8.0
1999-01-01	6.7
2000-01-01	5.4
2001-01-01	4.7
2002-01-01	4.7
2003-01-01	5.2
2004-01-01	6.1
2005-01-01	7.5
2006-01-01	7.1
2007-01-01	6.4
2008-01-01	6.3
2009-01-01	7.9
2010-01-01	8.4
2011-01-01	7.6
2012-01-01	7.6