Year	Percent
1971-01-01	3.6
1972-01-01	3.9
1973-01-01	3.0
1974-01-01	3.0
1975-01-01	4.1
1976-01-01	5.2
1977-01-01	5.3
1978-01-01	5.1
1979-01-01	4.9
1980-01-01	6.7
1981-01-01	10.3
1982-01-01	11.6
1983-01-01	12.2
1984-01-01	12.0
1985-01-01	11.8
1986-01-01	11.8
1987-01-01	10.9
1988-01-01	8.8
1989-01-01	7.4
1990-01-01	7.4
1991-01-01	9.9
1992-01-01	11.8
1993-01-01	12.4
1994-01-01	11.3
1995-01-01	10.1
1996-01-01	9.5
1997-01-01	7.9
1998-01-01	7.0
1999-01-01	6.6
2000-01-01	6.0
2001-01-01	5.7
2002-01-01	5.8
2003-01-01	5.6
2004-01-01	5.2
2005-01-01	5.3
2006-01-01	5.8
2007-01-01	5.7
2008-01-01	6.2
2009-01-01	8.7
2010-01-01	8.7
2011-01-01	8.8
2012-01-01	8.4