Year	Percent
1956-01-01	3.4
1957-01-01	4.6
1958-01-01	7.0
1959-01-01	6.0
1960-01-01	6.9
1961-01-01	7.2
1962-01-01	5.9
1963-01-01	5.5
1964-01-01	4.7
1965-01-01	3.9
1966-01-01	3.4
1967-01-01	3.8
1968-01-01	4.5
1969-01-01	4.4
1970-01-01	5.7
1971-01-01	6.2
1972-01-01	6.2
1973-01-01	5.6
1974-01-01	5.3
1975-01-01	6.9
1976-01-01	7.1
1977-01-01	8.0
1978-01-01	8.4
1979-01-01	7.5
1980-01-01	7.5
1981-01-01	7.6
1982-01-01	11.0
1983-01-01	12.0
1984-01-01	11.3
1985-01-01	10.6
1986-01-01	9.7
1987-01-01	8.8
1988-01-01	7.8
1989-01-01	7.5
1990-01-01	8.1
1991-01-01	10.3
1992-01-01	11.2
1993-01-01	11.4
1994-01-01	10.4
1995-01-01	9.5
1996-01-01	9.6
1997-01-01	9.1
1998-01-01	8.3
1999-01-01	7.6
2000-01-01	6.8
2001-01-01	7.2
2002-01-01	7.7
2003-01-01	7.6
2004-01-01	7.2
2005-01-01	6.8
2006-01-01	6.3
2007-01-01	6.0
2008-01-01	6.1
2009-01-01	8.3
2010-01-01	8.0
2011-01-01	7.4