Year	Percent
1970-01-01	2.9
1971-01-01	5.1
1972-01-01	5.8
1973-01-01	5.3
1974-01-01	4.5
1975-01-01	3.8
1976-01-01	3.8
1977-01-01	4.5
1978-01-01	5.6
1979-01-01	5.1
1980-01-01	5.1
1981-01-01	6.4
1982-01-01	7.7
1983-01-01	8.1
1984-01-01	6.1
1985-01-01	5.9
1986-01-01	5.7
1987-01-01	5.3
1988-01-01	4.3
1989-01-01	4.0
1990-01-01	4.6
1991-01-01	8.0
1992-01-01	13.9
1993-01-01	23.5
1994-01-01	23.8
1995-01-01	20.2
1996-01-01	21.5
1997-01-01	21.1
1998-01-01	17.1
1999-01-01	14.6
2000-01-01	12.0
2001-01-01	12.0
2002-01-01	13.1
2003-01-01	14.0
2004-01-01	17.2
2005-01-01	22.4
2006-01-01	21.4
2007-01-01	18.9
2008-01-01	19.4
2009-01-01	24.9
2010-01-01	24.5
2011-01-01	22.5
2012-01-01	23.3