Year	Percent
1970-01-01	2.5
1971-01-01	2.8
1972-01-01	2.9
1973-01-01	2.8
1974-01-01	2.9
1975-01-01	3.6
1976-01-01	4.0
1977-01-01	4.5
1978-01-01	4.6
1979-01-01	5.2
1980-01-01	5.6
1981-01-01	6.6
1982-01-01	6.9
1983-01-01	7.3
1984-01-01	8.5
1985-01-01	9.0
1986-01-01	9.0
1987-01-01	9.2
1988-01-01	8.9
1989-01-01	8.3
1990-01-01	8.0
1991-01-01	8.2
1992-01-01	9.1
1993-01-01	10.2
1994-01-01	10.8
1995-01-01	10.2
1996-01-01	10.7
1997-01-01	10.8
1998-01-01	10.4
1999-01-01	10.1
2000-01-01	8.6
2001-01-01	7.8
2002-01-01	8.0
2003-01-01	8.6
2004-01-01	9.0
2005-01-01	9.0
2006-01-01	8.9
2007-01-01	8.1
2008-01-01	7.5
2009-01-01	9.2
2010-01-01	9.4
2011-01-01	9.3
2012-01-01	10.0