Year	Percent
1970-01-01	1.1
1971-01-01	1.3
1972-01-01	1.9
1973-01-01	1.6
1974-01-01	1.9
1975-01-01	3.8
1976-01-01	3.9
1977-01-01	4.6
1978-01-01	5.4
1979-01-01	5.2
1980-01-01	5.1
1981-01-01	4.8
1982-01-01	6.4
1983-01-01	9.7
1984-01-01	8.7
1985-01-01	7.9
1986-01-01	7.7
1987-01-01	7.8
1988-01-01	6.8
1989-01-01	5.7
1990-01-01	6.7
1991-01-01	9.9
1992-01-01	11.4
1993-01-01	11.5
1994-01-01	10.0
1995-01-01	8.8
1996-01-01	8.8
1997-01-01	8.7
1998-01-01	8.0
1999-01-01	7.1
2000-01-01	6.5
2001-01-01	7.0
2002-01-01	6.5
2003-01-01	5.9
2004-01-01	5.3
2005-01-01	4.9
2006-01-01	4.7
2007-01-01	4.0
2008-01-01	4.0
2009-01-01	5.7
2010-01-01	5.1
2011-01-01	4.9
2012-01-01	5.2