Year	Percent
1970-01-01	1.0
1971-01-01	1.4
1972-01-01	2.4
1973-01-01	2.5
1974-01-01	3.0
1975-01-01	5.8
1976-01-01	6.2
1977-01-01	6.0
1978-01-01	5.7
1979-01-01	5.8
1980-01-01	4.3
1981-01-01	5.8
1982-01-01	7.7
1983-01-01	8.3
1984-01-01	8.1
1985-01-01	7.3
1986-01-01	6.5
1987-01-01	6.3
1988-01-01	6.2
1989-01-01	5.7
1990-01-01	5.1
1991-01-01	4.8
1992-01-01	4.9
1993-01-01	5.6
1994-01-01	6.2
1995-01-01	7.0
1996-01-01	6.4
1997-01-01	5.4
1998-01-01	4.3
1999-01-01	3.6
2000-01-01	3.0
2001-01-01	2.6
2002-01-01	3.1
2003-01-01	4.1
2004-01-01	5.1
2005-01-01	5.3
2006-01-01	4.3
2007-01-01	3.6
2008-01-01	3.1
2009-01-01	3.7
2010-01-01	4.5
2011-01-01	4.4