Year	Percent
1978-01-01	6.0
1979-01-01	7.7
1980-01-01	9.0
1981-01-01	11.0
1982-01-01	13.0
1983-01-01	14.4
1984-01-01	16.7
1985-01-01	17.8
1986-01-01	19.2
1987-01-01	18.5
1988-01-01	17.5
1989-01-01	15.5
1990-01-01	14.4
1991-01-01	14.5
1992-01-01	16.3
1993-01-01	20.1
1994-01-01	21.3
1995-01-01	20.1
1996-01-01	19.1
1997-01-01	17.8
1998-01-01	15.9
1999-01-01	13.3
2000-01-01	11.7
2001-01-01	10.5
2002-01-01	11.4
2003-01-01	11.4
2004-01-01	10.9
2005-01-01	9.2
2006-01-01	8.5
2007-01-01	8.3
2008-01-01	11.4
2009-01-01	18.0
2010-01-01	20.1
2011-01-01	21.7