Year	Percent
1970-01-01	1.9
1971-01-01	2.0
1972-01-01	2.1
1973-01-01	2.0
1974-01-01	2.1
1975-01-01	2.6
1976-01-01	2.8
1977-01-01	3.1
1978-01-01	3.3
1979-01-01	3.7
1980-01-01	3.9
1981-01-01	4.6
1982-01-01	4.9
1983-01-01	5.1
1984-01-01	5.9
1985-01-01	6.5
1986-01-01	6.9
1987-01-01	7.3
1988-01-01	7.3
1989-01-01	6.9
1990-01-01	6.7
1991-01-01	7.0
1992-01-01	7.8
1993-01-01	8.6
1994-01-01	9.2
1995-01-01	8.8
1996-01-01	9.3
1997-01-01	9.5
1998-01-01	9.2
1999-01-01	8.9
2000-01-01	7.7
2001-01-01	6.9
2002-01-01	6.9
2003-01-01	7.4
2004-01-01	7.6
2005-01-01	7.6
2006-01-01	7.4
2007-01-01	6.8
2008-01-01	6.2
2009-01-01	7.6
2010-01-01	7.9
2011-01-01	7.9
2012-01-01	8.4