Year	Percent
1955-01-01	4.4
1956-01-01	4.1
1957-01-01	4.3
1958-01-01	6.8
1959-01-01	5.5
1960-01-01	5.5
1961-01-01	6.7
1962-01-01	5.5
1963-01-01	5.7
1964-01-01	5.2
1965-01-01	4.5
1966-01-01	3.8
1967-01-01	3.8
1968-01-01	3.6
1969-01-01	3.5
1970-01-01	4.9
1971-01-01	6.0
1972-01-01	5.6
1973-01-01	4.9
1974-01-01	5.6
1975-01-01	8.5
1976-01-01	7.7
1977-01-01	7.1
1978-01-01	6.1
1979-01-01	5.8
1980-01-01	7.1
1981-01-01	7.6
1982-01-01	9.7
1983-01-01	9.6
1984-01-01	7.5
1985-01-01	7.2
1986-01-01	7.0
1987-01-01	6.2
1988-01-01	5.5
1989-01-01	5.3
1990-01-01	5.6
1991-01-01	6.8
1992-01-01	7.5
1993-01-01	6.9
1994-01-01	6.1
1995-01-01	5.6
1996-01-01	5.4
1997-01-01	5.0
1998-01-01	4.5
1999-01-01	4.2
2000-01-01	4.0
2001-01-01	4.7
2002-01-01	5.8
2003-01-01	6.0
2004-01-01	5.5
2005-01-01	5.1
2006-01-01	4.6
2007-01-01	4.6
2008-01-01	5.8
2009-01-01	9.3
2010-01-01	9.6
2011-01-01	9.0