Year	Percent
1973-01-01	25.3
1974-01-01	25.1
1975-01-01	24.5
1976-01-01	23.6
1977-01-01	23.2
1978-01-01	22.7
1979-01-01	22.0
1980-01-01	21.3
1981-01-01	20.8
1982-01-01	20.1
1983-01-01	19.6
1984-01-01	19.6
1985-01-01	19.5
1986-01-01	19.4
1987-01-01	19.4
1988-01-01	18.8
1989-01-01	19.1
1990-01-01	19.1
1991-01-01	18.3
1992-01-01	18.1
1993-01-01	17.7
1994-01-01	16.7
1995-01-01	16.4
1996-01-01	15.9
1997-01-01	15.7
1998-01-01	15.2
1999-01-01	15.1
2000-01-01	14.8
2001-01-01	14.5
2002-01-01	13.9
2003-01-01	13.5
2004-01-01	13.8
2005-01-01	13.8
2006-01-01	13.4
2007-01-01	12.6
2008-01-01	11.4
2009-01-01	10.8
2010-01-01	10.6
2011-01-01	10.3
2012-01-01	10.2