Year	Percent
1978-01-01	17.2
1979-01-01	18.1
1980-01-01	17.1
1981-01-01	15.6
1982-01-01	18.4
1983-01-01	23.6
1984-01-01	22.3
1985-01-01	20.3
1986-01-01	20.1
1987-01-01	19.9
1988-01-01	17.4
1989-01-01	14.8
1990-01-01	16.9
1991-01-01	22.3
1992-01-01	24.6
1993-01-01	23.8
1994-01-01	22.7
1995-01-01	20.6
1996-01-01	20.8
1997-01-01	20.3
1998-01-01	19.2
1999-01-01	17.6
2000-01-01	16.1
2001-01-01	17.7
2002-01-01	16.8
2003-01-01	15.7
2004-01-01	15.6
2005-01-01	14.9
2006-01-01	14.8
2007-01-01	13.8
2008-01-01	12.8
2009-01-01	16.5
2010-01-01	16.8
2011-01-01	16.1
2012-01-01	16.7