Year	Percent
1970-01-01	9.0
1971-01-01	8.9
1972-01-01	10.9
1973-01-01	11.0
1974-01-01	9.8
1975-01-01	10.6
1976-01-01	12.2
1977-01-01	12.0
1978-01-01	12.6
1979-01-01	12.7
1980-01-01	12.9
1981-01-01	14.3
1982-01-01	15.8
1983-01-01	17.0
1984-01-01	17.7
1985-01-01	18.5
1986-01-01	22.0
1987-01-01	22.5
1988-01-01	22.1
1989-01-01	21.5
1990-01-01	19.0
1991-01-01	19.2
1992-01-01	20.5
1993-01-01	25.3
1994-01-01	26.9
1995-01-01	28.5
1996-01-01	28.5
1997-01-01	28.3
1998-01-01	27.5
1999-01-01	26.4
2000-01-01	24.7
2001-01-01	21.7
2002-01-01	20.7
2003-01-01	22.2
2004-01-01	21.1
2005-01-01	21.4
2006-01-01	19.6
2007-01-01	18.2
2008-01-01	18.7
2009-01-01	23.0
2010-01-01	25.0
2011-01-01	26.3
2012-01-01	32.1