Year	Percent
1970-01-01	2.0
1971-01-01	2.0
1972-01-01	2.3
1973-01-01	2.1
1974-01-01	2.3
1975-01-01	2.9
1976-01-01	2.9
1977-01-01	3.4
1978-01-01	3.6
1979-01-01	3.4
1980-01-01	3.3
1981-01-01	3.6
1982-01-01	4.0
1983-01-01	4.1
1984-01-01	4.4
1985-01-01	4.0
1986-01-01	4.5
1987-01-01	4.3
1988-01-01	4.1
1989-01-01	3.7
1990-01-01	3.6
1991-01-01	3.7
1992-01-01	3.8
1993-01-01	4.6
1994-01-01	4.7
1995-01-01	5.5
1996-01-01	5.9
1997-01-01	5.9
1998-01-01	6.8
1999-01-01	8.0
2000-01-01	8.3
2001-01-01	8.5
2002-01-01	8.9
2003-01-01	9.1
2004-01-01	8.5
2005-01-01	7.8
2006-01-01	7.2
2007-01-01	7.4
2008-01-01	6.9
2009-01-01	8.9
2010-01-01	9.0
2011-01-01	7.9
2012-01-01	7.5