Year	Percent
1970-01-01	20.1
1971-01-01	20.1
1972-01-01	19.0
1973-01-01	18.2
1974-01-01	17.4
1975-01-01	16.7
1976-01-01	16.4
1977-01-01	15.8
1978-01-01	15.4
1979-01-01	14.9
1980-01-01	14.2
1981-01-01	13.3
1982-01-01	12.4
1983-01-01	12.3
1984-01-01	11.7
1985-01-01	11.0
1986-01-01	10.7
1987-01-01	10.4
1988-01-01	9.8
1989-01-01	9.3
1990-01-01	8.9
1991-01-01	8.5
1992-01-01	8.2
1993-01-01	6.4
1994-01-01	6.2
1995-01-01	5.9
1996-01-01	5.6
1997-01-01	5.4
1998-01-01	5.2
1999-01-01	4.9
2000-01-01	4.7
2001-01-01	4.6
2002-01-01	4.5
2003-01-01	4.4
2004-01-01	4.5
2005-01-01	4.2
2006-01-01	4.3
2007-01-01	4.0
2008-01-01	3.7
2009-01-01	3.7
2010-01-01	3.8
2011-01-01	3.7
2012-01-01	3.7