Year	Percent
1970-01-01	2.8
1971-01-01	3.1
1972-01-01	3.9
1973-01-01	3.6
1974-01-01	4.1
1975-01-01	7.0
1976-01-01	6.4
1977-01-01	7.5
1978-01-01	7.9
1979-01-01	8.2
1980-01-01	7.9
1981-01-01	7.4
1982-01-01	8.5
1983-01-01	10.4
1984-01-01	9.5
1985-01-01	8.8
1986-01-01	8.7
1987-01-01	8.6
1988-01-01	7.9
1989-01-01	6.8
1990-01-01	7.2
1991-01-01	9.2
1992-01-01	10.0
1993-01-01	10.1
1994-01-01	9.4
1995-01-01	8.1
1996-01-01	8.2
1997-01-01	8.2
1998-01-01	7.4
1999-01-01	6.7
2000-01-01	6.1
2001-01-01	6.4
2002-01-01	6.2
2003-01-01	6.0
2004-01-01	5.5
2005-01-01	5.2
2006-01-01	4.9
2007-01-01	4.8
2008-01-01	4.6
2009-01-01	5.4
2010-01-01	5.4
2011-01-01	5.3
2012-01-01	5.3