Year	Percent
1973-01-01	3.8
1974-01-01	5.1
1975-01-01	8.2
1976-01-01	9.3
1977-01-01	8.9
1978-01-01	9.6
1979-01-01	10.2
1980-01-01	11.9
1981-01-01	16.8
1982-01-01	19.8
1983-01-01	20.9
1984-01-01	20.8
1985-01-01	17.4
1986-01-01	16.8
1987-01-01	15.2
1988-01-01	14.1
1989-01-01	12.5
1990-01-01	11.5
1991-01-01	10.7
1992-01-01	9.7
1993-01-01	9.9
1994-01-01	10.4
1995-01-01	12.9
1996-01-01	12.1
1997-01-01	9.5
1998-01-01	8.2
1999-01-01	7.1
2000-01-01	6.0
2001-01-01	5.0
2002-01-01	5.5
2003-01-01	7.0
2004-01-01	8.6
2005-01-01	9.0
2006-01-01	7.2
2007-01-01	6.8
2008-01-01	6.2
2009-01-01	7.5
2010-01-01	8.8
2011-01-01	7.7
2012-01-01	9.6