Year	Percent
1970-01-01	26.4
1971-01-01	24.7
1972-01-01	24.3
1973-01-01	24.8
1974-01-01	24.2
1975-01-01	22.7
1976-01-01	22.8
1977-01-01	22.7
1978-01-01	22.7
1979-01-01	22.7
1980-01-01	22.1
1981-01-01	21.7
1982-01-01	20.4
1983-01-01	18.4
1984-01-01	18.5
1985-01-01	18.1
1986-01-01	17.8
1987-01-01	17.3
1988-01-01	17.3
1989-01-01	17.2
1990-01-01	16.8
1991-01-01	16.3
1992-01-01	15.9
1993-01-01	15.4
1994-01-01	15.4
1995-01-01	15.5
1996-01-01	15.3
1997-01-01	15.2
1998-01-01	15.0
1999-01-01	14.3
2000-01-01	14.4
2001-01-01	13.5
2002-01-01	12.6
2003-01-01	12.3
2004-01-01	11.8
2005-01-01	11.5
2006-01-01	11.3
2007-01-01	11.2
2008-01-01	10.9
2009-01-01	10.2
2010-01-01	10.1
2011-01-01	10.2
2012-01-01	10.3