Year	Percent
1970-01-01	0.6
1971-01-01	0.8
1972-01-01	0.8
1973-01-01	0.9
1974-01-01	1.8
1975-01-01	3.5
1976-01-01	3.9
1977-01-01	4.3
1978-01-01	4.2
1979-01-01	3.9
1980-01-01	3.6
1981-01-01	4.8
1982-01-01	6.5
1983-01-01	7.9
1984-01-01	8.5
1985-01-01	8.6
1986-01-01	8.1
1987-01-01	7.8
1988-01-01	7.8
1989-01-01	7.2
1990-01-01	6.1
1991-01-01	7.0
1992-01-01	8.4
1993-01-01	9.6
1994-01-01	10.1
1995-01-01	9.4
1996-01-01	9.6
1997-01-01	10.6
1998-01-01	9.8
1999-01-01	8.9
2000-01-01	8.1
2001-01-01	7.9
2002-01-01	8.3
2003-01-01	8.8
2004-01-01	10.0
2005-01-01	10.9
2006-01-01	10.2
2007-01-01	8.8
2008-01-01	7.7
2009-01-01	7.3
2010-01-01	6.5
2011-01-01	5.6
2012-01-01	5.2