Year	Percent
1970-01-01	22.3
1971-01-01	21.8
1972-01-01	21.8
1973-01-01	22.0
1974-01-01	21.7
1975-01-01	20.2
1976-01-01	19.2
1977-01-01	18.8
1978-01-01	18.9
1979-01-01	19.2
1980-01-01	19.1
1981-01-01	18.5
1982-01-01	17.2
1983-01-01	16.8
1984-01-01	17.1
1985-01-01	17.0
1986-01-01	17.1
1987-01-01	16.7
1988-01-01	16.6
1989-01-01	16.5
1990-01-01	15.8
1991-01-01	14.8
1992-01-01	14.3
1993-01-01	14.0
1994-01-01	14.0
1995-01-01	14.4
1996-01-01	14.4
1997-01-01	14.8
1998-01-01	15.0
1999-01-01	15.3
2000-01-01	15.3
2001-01-01	15.0
2002-01-01	15.0
2003-01-01	14.6
2004-01-01	14.5
2005-01-01	13.8
2006-01-01	12.9
2007-01-01	12.1
2008-01-01	11.5
2009-01-01	10.6
2010-01-01	10.3
2011-01-01	10.2
2012-01-01	10.2