Year	Percent
1970-01-01	3.2
1971-01-01	3.3
1972-01-01	3.8
1973-01-01	3.7
1974-01-01	3.1
1975-01-01	3.4
1976-01-01	3.9
1977-01-01	4.1
1978-01-01	4.1
1979-01-01	4.4
1980-01-01	4.4
1981-01-01	4.9
1982-01-01	5.4
1983-01-01	5.9
1984-01-01	5.9
1985-01-01	6.0
1986-01-01	7.5
1987-01-01	7.9
1988-01-01	7.9
1989-01-01	7.8
1990-01-01	7.0
1991-01-01	6.9
1992-01-01	7.3
1993-01-01	9.8
1994-01-01	10.7
1995-01-01	11.3
1996-01-01	11.3
1997-01-01	11.3
1998-01-01	11.4
1999-01-01	11.0
2000-01-01	10.1
2001-01-01	9.1
2002-01-01	8.6
2003-01-01	8.5
2004-01-01	8.1
2005-01-01	7.8
2006-01-01	6.9
2007-01-01	6.2
2008-01-01	6.8
2009-01-01	7.9
2010-01-01	8.5
2011-01-01	8.5
2012-01-01	10.8