Year	Percent
1973-01-01	3.1
1974-01-01	3.6
1975-01-01	5.1
1976-01-01	5.4
1977-01-01	4.9
1978-01-01	5.1
1979-01-01	5.1
1980-01-01	6.0
1981-01-01	8.9
1982-01-01	10.2
1983-01-01	11.4
1984-01-01	11.5
1985-01-01	9.6
1986-01-01	10.0
1987-01-01	9.8
1988-01-01	9.3
1989-01-01	8.4
1990-01-01	7.6
1991-01-01	7.1
1992-01-01	6.8
1993-01-01	6.3
1994-01-01	6.9
1995-01-01	7.1
1996-01-01	6.6
1997-01-01	5.6
1998-01-01	4.4
1999-01-01	3.5
2000-01-01	3.1
2001-01-01	2.5
2002-01-01	3.0
2003-01-01	4.1
2004-01-01	5.0
2005-01-01	5.2
2006-01-01	4.3
2007-01-01	3.5
2008-01-01	3.0
2009-01-01	3.7
2010-01-01	4.5
2011-01-01	4.5
2012-01-01	5.3