Year	Percent
1970-01-01	0.5
1971-01-01	0.6
1972-01-01	0.7
1973-01-01	0.7
1974-01-01	1.6
1975-01-01	3.4
1976-01-01	3.4
1977-01-01	3.4
1978-01-01	3.3
1979-01-01	2.9
1980-01-01	2.8
1981-01-01	4.0
1982-01-01	5.6
1983-01-01	6.9
1984-01-01	7.1
1985-01-01	7.2
1986-01-01	6.6
1987-01-01	6.3
1988-01-01	6.3
1989-01-01	5.7
1990-01-01	5.0
1991-01-01	5.6
1992-01-01	6.7
1993-01-01	8.0
1994-01-01	8.5
1995-01-01	8.2
1996-01-01	9.0
1997-01-01	9.9
1998-01-01	9.3
1999-01-01	8.5
2000-01-01	7.8
2001-01-01	7.9
2002-01-01	8.6
2003-01-01	9.3
2004-01-01	10.3
2005-01-01	11.2
2006-01-01	10.3
2007-01-01	8.7
2008-01-01	7.6
2009-01-01	7.8
2010-01-01	7.1
2011-01-01	5.9
2012-01-01	5.5