Year	Percent
1970-01-01	2.3
1971-01-01	2.4
1972-01-01	3.0
1973-01-01	3.1
1974-01-01	3.4
1975-01-01	5.6
1976-01-01	7.2
1977-01-01	8.2
1978-01-01	8.8
1979-01-01	9.1
1980-01-01	7.0
1981-01-01	8.5
1982-01-01	9.8
1983-01-01	10.8
1984-01-01	10.8
1985-01-01	10.1
1986-01-01	10.1
1987-01-01	9.8
1988-01-01	8.8
1989-01-01	7.4
1990-01-01	6.6
1991-01-01	6.4
1992-01-01	7.1
1993-01-01	8.6
1994-01-01	9.8
1995-01-01	9.7
1996-01-01	9.6
1997-01-01	9.2
1998-01-01	9.3
1999-01-01	8.5
2000-01-01	6.9
2001-01-01	6.6
2002-01-01	7.5
2003-01-01	8.2
2004-01-01	8.4
2005-01-01	8.5
2006-01-01	8.2
2007-01-01	7.5
2008-01-01	7.0
2009-01-01	7.9
2010-01-01	8.3
2011-01-01	7.2