Year	Percent
1970-01-01	4.2
1971-01-01	4.6
1972-01-01	4.7
1973-01-01	4.6
1974-01-01	4.7
1975-01-01	5.0
1976-01-01	5.8
1977-01-01	6.4
1978-01-01	6.5
1979-01-01	7.3
1980-01-01	8.1
1981-01-01	9.1
1982-01-01	9.1
1983-01-01	9.3
1984-01-01	10.4
1985-01-01	10.8
1986-01-01	10.8
1987-01-01	11.4
1988-01-01	11.2
1989-01-01	10.7
1990-01-01	10.2
1991-01-01	10.4
1992-01-01	11.2
1993-01-01	11.8
1994-01-01	12.4
1995-01-01	11.9
1996-01-01	12.2
1997-01-01	12.3
1998-01-01	11.9
1999-01-01	11.5
2000-01-01	10.1
2001-01-01	9.2
2002-01-01	8.9
2003-01-01	9.5
2004-01-01	9.9
2005-01-01	9.9
2006-01-01	9.7
2007-01-01	8.6
2008-01-01	7.9
2009-01-01	9.4
2010-01-01	9.7
2011-01-01	9.7
2012-01-01	10.0