Year	Percent
1970-01-01	1.5
1971-01-01	2.5
1972-01-01	2.7
1973-01-01	2.4
1974-01-01	2.0
1975-01-01	1.6
1976-01-01	1.6
1977-01-01	1.8
1978-01-01	2.2
1979-01-01	2.1
1980-01-01	2.1
1981-01-01	2.6
1982-01-01	3.3
1983-01-01	3.7
1984-01-01	3.3
1985-01-01	2.9
1986-01-01	2.7
1987-01-01	2.2
1988-01-01	1.8
1989-01-01	1.5
1990-01-01	1.7
1991-01-01	3.1
1992-01-01	5.6
1993-01-01	9.0
1994-01-01	9.3
1995-01-01	8.8
1996-01-01	9.5
1997-01-01	9.9
1998-01-01	8.2
1999-01-01	6.7
2000-01-01	5.6
2001-01-01	5.9
2002-01-01	6.0
2003-01-01	6.6
2004-01-01	7.4
2005-01-01	7.7
2006-01-01	7.1
2007-01-01	6.1
2008-01-01	6.2
2009-01-01	8.3
2010-01-01	8.4
2011-01-01	7.5