Year	Percent
1970-01-01	2.0
1971-01-01	2.2
1972-01-01	3.0
1973-01-01	2.8
1974-01-01	2.6
1975-01-01	3.6
1976-01-01	4.0
1977-01-01	4.1
1978-01-01	4.6
1979-01-01	4.8
1980-01-01	4.1
1981-01-01	5.6
1982-01-01	5.6
1983-01-01	6.3
1984-01-01	6.9
1985-01-01	7.2
1986-01-01	7.3
1987-01-01	7.8
1988-01-01	7.2
1989-01-01	7.0
1990-01-01	6.6
1991-01-01	6.5
1992-01-01	6.7
1993-01-01	7.1
1994-01-01	7.4
1995-01-01	8.1
1996-01-01	8.9
1997-01-01	8.9
1998-01-01	10.5
1999-01-01	12.2
2000-01-01	11.8
2001-01-01	11.9
2002-01-01	12.5
2003-01-01	11.6
2004-01-01	11.4
2005-01-01	9.7
2006-01-01	9.1
2007-01-01	8.7
2008-01-01	7.8
2009-01-01	9.5
2010-01-01	9.9
2011-01-01	9.0
2012-01-01	7.6