Year	Percent
1970-01-01	4.3
1971-01-01	7.8
1972-01-01	8.3
1973-01-01	6.9
1974-01-01	6.6
1975-01-01	5.6
1976-01-01	5.5
1977-01-01	6.9
1978-01-01	8.0
1979-01-01	7.5
1980-01-01	7.6
1981-01-01	9.5
1982-01-01	10.7
1983-01-01	10.5
1984-01-01	5.0
1985-01-01	4.6
1986-01-01	4.2
1987-01-01	5.9
1988-01-01	5.3
1989-01-01	5.4
1990-01-01	7.1
1991-01-01	10.7
1992-01-01	15.8
1993-01-01	26.2
1994-01-01	25.0
1995-01-01	21.2
1996-01-01	22.8
1997-01-01	24.3
1998-01-01	20.8
1999-01-01	18.3
2000-01-01	17.8
2001-01-01	16.8
2002-01-01	18.7
2003-01-01	19.1
2004-01-01	22.7
2005-01-01	32.7
2006-01-01	30.9
2007-01-01	29.6
2008-01-01	30.4
2009-01-01	35.9
2010-01-01	35.6
2011-01-01	33.9
2012-01-01	35.8