Year	Percent
1970-01-01	0.4
1971-01-01	0.5
1972-01-01	0.6
1973-01-01	0.8
1974-01-01	2.3
1975-01-01	4.5
1976-01-01	4.6
1977-01-01	4.8
1978-01-01	4.1
1979-01-01	3.2
1980-01-01	3.4
1981-01-01	5.4
1982-01-01	7.8
1983-01-01	8.6
1984-01-01	12.8
1985-01-01	11.4
1986-01-01	8.9
1987-01-01	7.8
1988-01-01	7.6
1989-01-01	6.5
1990-01-01	5.1
1991-01-01	5.0
1992-01-01	5.0
1993-01-01	5.6
1994-01-01	6.0
1995-01-01	7.0
1996-01-01	8.1
1997-01-01	9.0
1998-01-01	8.2
1999-01-01	7.7
2000-01-01	8.1
2001-01-01	7.1
2002-01-01	8.1
2003-01-01	7.6
2004-01-01	9.9
2005-01-01	14.9
2006-01-01	14.2
2007-01-01	12.9
2008-01-01	11.0
2009-01-01	11.3
2010-01-01	10.7
2011-01-01	10.0
2012-01-01	9.2