Year	Percent
1970-01-01	11.0
1971-01-01	12.7
1972-01-01	12.1
1973-01-01	10.5
1974-01-01	11.9
1975-01-01	16.1
1976-01-01	14.7
1977-01-01	13.6
1978-01-01	12.3
1979-01-01	11.8
1980-01-01	13.9
1981-01-01	14.9
1982-01-01	17.8
1983-01-01	17.2
1984-01-01	13.9
1985-01-01	13.6
1986-01-01	13.3
1987-01-01	12.2
1988-01-01	11.0
1989-01-01	10.9
1990-01-01	11.2
1991-01-01	13.4
1992-01-01	14.2
1993-01-01	13.4
1994-01-01	12.5
1995-01-01	12.1
1996-01-01	12.0
1997-01-01	11.3
1998-01-01	10.4
1999-01-01	9.9
2000-01-01	9.3
2001-01-01	10.6
2002-01-01	12.0
2003-01-01	12.4
2004-01-01	11.8
2005-01-01	11.3
2006-01-01	10.5
2007-01-01	10.5
2008-01-01	12.8
2009-01-01	17.6
2010-01-01	18.4
2011-01-01	17.3
2012-01-01	16.2