Year	Percent
1970-01-01	2.5
1971-01-01	2.5
1972-01-01	2.9
1973-01-01	2.7
1974-01-01	2.3
1975-01-01	2.5
1976-01-01	2.7
1977-01-01	2.8
1978-01-01	2.8
1979-01-01	3.0
1980-01-01	2.9
1981-01-01	3.3
1982-01-01	3.8
1983-01-01	4.1
1984-01-01	4.2
1985-01-01	4.2
1986-01-01	5.2
1987-01-01	5.5
1988-01-01	5.5
1989-01-01	5.4
1990-01-01	4.8
1991-01-01	4.9
1992-01-01	5.3
1993-01-01	7.3
1994-01-01	8.3
1995-01-01	8.7
1996-01-01	8.7
1997-01-01	8.7
1998-01-01	8.8
1999-01-01	8.4
2000-01-01	7.8
2001-01-01	7.0
2002-01-01	6.6
2003-01-01	6.6
2004-01-01	6.5
2005-01-01	6.3
2006-01-01	5.5
2007-01-01	5.0
2008-01-01	5.6
2009-01-01	6.9
2010-01-01	7.7
2011-01-01	7.7
2012-01-01	10.0