Year	Percent
1970-01-01	2.0
1971-01-01	2.1
1972-01-01	2.5
1973-01-01	2.3
1974-01-01	2.4
1975-01-01	3.1
1976-01-01	3.2
1977-01-01	3.6
1978-01-01	3.8
1979-01-01	3.7
1980-01-01	3.5
1981-01-01	4.0
1982-01-01	4.3
1983-01-01	4.6
1984-01-01	4.9
1985-01-01	4.7
1986-01-01	5.1
1987-01-01	5.0
1988-01-01	4.7
1989-01-01	4.4
1990-01-01	4.3
1991-01-01	4.2
1992-01-01	4.4
1993-01-01	5.1
1994-01-01	5.2
1995-01-01	5.9
1996-01-01	6.4
1997-01-01	6.4
1998-01-01	7.4
1999-01-01	8.7
2000-01-01	8.9
2001-01-01	9.1
2002-01-01	9.5
2003-01-01	9.6
2004-01-01	9.0
2005-01-01	8.1
2006-01-01	7.5
2007-01-01	7.6
2008-01-01	7.0
2009-01-01	9.0
2010-01-01	9.2
2011-01-01	8.1
2012-01-01	7.5