Year	Percent
1970-01-01	1.5
1971-01-01	1.7
1972-01-01	1.8
1973-01-01	1.6
1974-01-01	1.8
1975-01-01	2.7
1976-01-01	2.8
1977-01-01	3.2
1978-01-01	3.4
1979-01-01	3.9
1980-01-01	3.9
1981-01-01	4.9
1982-01-01	5.4
1983-01-01	5.8
1984-01-01	7.0
1985-01-01	7.6
1986-01-01	7.6
1987-01-01	7.5
1988-01-01	7.1
1989-01-01	6.4
1990-01-01	6.3
1991-01-01	6.5
1992-01-01	7.4
1993-01-01	8.8
1994-01-01	9.5
1995-01-01	8.7
1996-01-01	9.4
1997-01-01	9.6
1998-01-01	9.1
1999-01-01	8.9
2000-01-01	7.3
2001-01-01	6.6
2002-01-01	7.2
2003-01-01	7.8
2004-01-01	8.2
2005-01-01	8.2
2006-01-01	8.2
2007-01-01	7.6
2008-01-01	7.0
2009-01-01	9.1
2010-01-01	9.1
2011-01-01	8.9
2012-01-01	9.9