Year	Percent
1970-01-01	27.7
1971-01-01	27.4
1972-01-01	27.2
1973-01-01	27.6
1974-01-01	28.4
1975-01-01	28.1
1976-01-01	27.0
1977-01-01	26.0
1978-01-01	25.0
1979-01-01	24.7
1980-01-01	24.3
1981-01-01	23.4
1982-01-01	22.5
1983-01-01	22.4
1984-01-01	22.5
1985-01-01	22.6
1986-01-01	22.8
1987-01-01	22.0
1988-01-01	21.7
1989-01-01	21.7
1990-01-01	21.0
1991-01-01	19.9
1992-01-01	18.9
1993-01-01	18.3
1994-01-01	18.2
1995-01-01	19.0
1996-01-01	19.3
1997-01-01	19.2
1998-01-01	19.1
1999-01-01	18.5
2000-01-01	18.0
2001-01-01	17.5
2002-01-01	16.8
2003-01-01	16.2
2004-01-01	16.0
2005-01-01	15.0
2006-01-01	14.7
2007-01-01	14.3
2008-01-01	13.9
2009-01-01	12.5
2010-01-01	12.1
2011-01-01	12.1
2012-01-01	11.7