Year	Percent
1964-01-01	1.5
1965-01-01	1.4
1966-01-01	1.5
1967-01-01	3.1
1968-01-01	3.9
1969-01-01	2.8
1970-01-01	2.6
1971-01-01	3.0
1972-01-01	3.3
1973-01-01	3.1
1974-01-01	2.5
1975-01-01	2.9
1976-01-01	4.5
1977-01-01	6.6
1978-01-01	7.9
1979-01-01	6.5
1980-01-01	5.3
1981-01-01	5.7
1982-01-01	6.1
1983-01-01	6.1
1984-01-01	5.9
1985-01-01	6.0
1986-01-01	6.7
1987-01-01	4.9
1988-01-01	4.2
1989-01-01	3.1
1990-01-01	3.2
1991-01-01	6.7
1992-01-01	11.6
1993-01-01	16.2
1994-01-01	16.8
1995-01-01	15.1
1996-01-01	14.9
1997-01-01	12.7
1998-01-01	11.4
1999-01-01	10.3
2000-01-01	9.6
2001-01-01	9.1
2002-01-01	9.1
2003-01-01	9.1
2004-01-01	8.9
2005-01-01	8.3
2006-01-01	7.7
2007-01-01	6.9
2008-01-01	6.4
2009-01-01	8.2
2010-01-01	8.4
2011-01-01	7.8