Year	Percent
1976-01-01	15.9
1977-01-01	17.5
1978-01-01	17.9
1979-01-01	16.3
1980-01-01	16.3
1981-01-01	16.2
1982-01-01	22.1
1983-01-01	22.4
1984-01-01	20.2
1985-01-01	18.6
1986-01-01	17.0
1987-01-01	14.9
1988-01-01	12.9
1989-01-01	12.5
1990-01-01	13.8
1991-01-01	16.3
1992-01-01	19.0
1993-01-01	19.3
1994-01-01	17.5
1995-01-01	17.1
1996-01-01	18.8
1997-01-01	20.8
1998-01-01	19.7
1999-01-01	17.8
2000-01-01	15.4
2001-01-01	15.6
2002-01-01	17.2
2003-01-01	17.4
2004-01-01	17.2
2005-01-01	15.4
2006-01-01	14.6
2007-01-01	13.4
2008-01-01	14.2
2009-01-01	18.5
2010-01-01	18.6
2011-01-01	17.7
2012-01-01	18.7