Year	Percent
1971-01-01	28.5
1972-01-01	28.8
1973-01-01	28.9
1974-01-01	28.8
1975-01-01	28.5
1976-01-01	28.0
1977-01-01	27.7
1978-01-01	27.2
1979-01-01	26.7
1980-01-01	26.3
1981-01-01	25.8
1982-01-01	24.4
1983-01-01	24.1
1984-01-01	23.7
1985-01-01	23.0
1986-01-01	22.3
1987-01-01	21.8
1988-01-01	21.4
1989-01-01	21.2
1990-01-01	21.3
1991-01-01	21.1
1992-01-01	20.4
1993-01-01	19.7
1994-01-01	19.3
1995-01-01	19.0
1996-01-01	18.7
1997-01-01	18.6
1998-01-01	18.4
1999-01-01	18.2
2000-01-01	17.9
2001-01-01	17.8
2002-01-01	17.4
2003-01-01	17.0
2004-01-01	16.7
2005-01-01	16.4
2006-01-01	16.1
2007-01-01	15.7
2008-01-01	14.6
2009-01-01	13.9
2010-01-01	13.3
2011-01-01	13.4
2012-01-01	13.0