Year	Percent
1970-01-01	0.6
1971-01-01	0.6
1972-01-01	0.7
1973-01-01	0.7
1974-01-01	1.4
1975-01-01	3.0
1976-01-01	3.0
1977-01-01	3.0
1978-01-01	3.0
1979-01-01	2.7
1980-01-01	2.6
1981-01-01	3.5
1982-01-01	5.0
1983-01-01	6.1
1984-01-01	6.1
1985-01-01	6.4
1986-01-01	6.2
1987-01-01	6.1
1988-01-01	6.2
1989-01-01	5.7
1990-01-01	5.0
1991-01-01	5.7
1992-01-01	6.8
1993-01-01	8.0
1994-01-01	8.5
1995-01-01	8.2
1996-01-01	8.9
1997-01-01	9.8
1998-01-01	9.3
1999-01-01	8.4
2000-01-01	7.7
2001-01-01	7.8
2002-01-01	8.4
2003-01-01	9.1
2004-01-01	10.0
2005-01-01	10.7
2006-01-01	9.9
2007-01-01	8.3
2008-01-01	7.2
2009-01-01	7.3
2010-01-01	6.8
2011-01-01	5.6
2012-01-01	5.2