Year	Percent
1970-01-01	15.3
1971-01-01	16.9
1972-01-01	16.2
1973-01-01	14.5
1974-01-01	16.0
1975-01-01	19.9
1976-01-01	19.0
1977-01-01	17.8
1978-01-01	16.4
1979-01-01	16.1
1980-01-01	17.8
1981-01-01	19.6
1982-01-01	23.2
1983-01-01	22.4
1984-01-01	18.9
1985-01-01	18.6
1986-01-01	18.3
1987-01-01	16.9
1988-01-01	15.3
1989-01-01	15.0
1990-01-01	15.5
1991-01-01	18.7
1992-01-01	20.1
1993-01-01	19.0
1994-01-01	17.6
1995-01-01	17.3
1996-01-01	16.7
1997-01-01	16.0
1998-01-01	14.6
1999-01-01	13.9
2000-01-01	13.1
2001-01-01	14.7
2002-01-01	16.5
2003-01-01	17.5
2004-01-01	17.0
2005-01-01	16.6
2006-01-01	15.4
2007-01-01	15.7
2008-01-01	18.7
2009-01-01	24.3
2010-01-01	25.9
2011-01-01	24.4
2012-01-01	24.0