Year	Percent
1970-01-01	0.5
1971-01-01	0.5
1972-01-01	0.7
1973-01-01	0.6
1974-01-01	1.5
1975-01-01	3.3
1976-01-01	3.0
1977-01-01	2.9
1978-01-01	2.7
1979-01-01	2.3
1980-01-01	2.3
1981-01-01	3.4
1982-01-01	5.0
1983-01-01	6.2
1984-01-01	6.2
1985-01-01	6.2
1986-01-01	5.6
1987-01-01	5.4
1988-01-01	5.3
1989-01-01	4.6
1990-01-01	4.2
1991-01-01	4.6
1992-01-01	5.5
1993-01-01	6.7
1994-01-01	7.3
1995-01-01	7.3
1996-01-01	8.4
1997-01-01	9.4
1998-01-01	8.9
1999-01-01	8.1
2000-01-01	7.6
2001-01-01	7.8
2002-01-01	8.8
2003-01-01	9.7
2004-01-01	10.6
2005-01-01	11.5
2006-01-01	10.4
2007-01-01	8.6
2008-01-01	7.5
2009-01-01	8.2
2010-01-01	7.6
2011-01-01	6.2
2012-01-01	5.7