Year	Percent
1970-01-01	4.9
1971-01-01	5.5
1972-01-01	5.9
1973-01-01	5.8
1974-01-01	6.3
1975-01-01	7.9
1976-01-01	9.0
1977-01-01	10.6
1978-01-01	11.1
1979-01-01	12.7
1980-01-01	14.2
1981-01-01	16.8
1982-01-01	15.5
1983-01-01	16.5
1984-01-01	20.1
1985-01-01	20.2
1986-01-01	19.1
1987-01-01	18.2
1988-01-01	17.1
1989-01-01	15.4
1990-01-01	15.5
1991-01-01	16.1
1992-01-01	17.7
1993-01-01	20.7
1994-01-01	22.4
1995-01-01	20.5
1996-01-01	21.8
1997-01-01	22.2
1998-01-01	20.9
1999-01-01	20.6
2000-01-01	16.4
2001-01-01	15.7
2002-01-01	16.6
2003-01-01	18.7
2004-01-01	20.3
2005-01-01	20.9
2006-01-01	21.9
2007-01-01	19.4
2008-01-01	18.8
2009-01-01	23.5
2010-01-01	23.0
2011-01-01	22.2
2012-01-01	23.9