Year	Percent
1955-01-01	2.5
1956-01-01	2.3
1957-01-01	1.9
1958-01-01	2.0
1959-01-01	2.2
1960-01-01	1.6
1961-01-01	1.4
1962-01-01	1.3
1963-01-01	1.3
1964-01-01	1.2
1965-01-01	1.2
1966-01-01	1.3
1967-01-01	1.3
1968-01-01	1.2
1969-01-01	1.1
1970-01-01	1.2
1971-01-01	1.2
1972-01-01	1.4
1973-01-01	1.3
1974-01-01	1.4
1975-01-01	1.9
1976-01-01	2.0
1977-01-01	2.0
1978-01-01	2.2
1979-01-01	2.1
1980-01-01	2.0
1981-01-01	2.2
1982-01-01	2.4
1983-01-01	2.7
1984-01-01	2.7
1985-01-01	2.6
1986-01-01	2.8
1987-01-01	2.8
1988-01-01	2.5
1989-01-01	2.3
1990-01-01	2.1
1991-01-01	2.1
1992-01-01	2.2
1993-01-01	2.5
1994-01-01	2.9
1995-01-01	3.1
1996-01-01	3.4
1997-01-01	3.4
1998-01-01	4.1
1999-01-01	4.7
2000-01-01	4.7
2001-01-01	5.0
2002-01-01	5.4
2003-01-01	5.3
2004-01-01	4.7
2005-01-01	4.4
2006-01-01	4.1
2007-01-01	3.9
2008-01-01	4.0
2009-01-01	5.1
2010-01-01	5.1
2011-01-01	4.6