Year	Percent
1970-01-01	34.6
1971-01-01	34.4
1972-01-01	33.5
1973-01-01	33.4
1974-01-01	33.0
1975-01-01	31.5
1976-01-01	31.0
1977-01-01	30.4
1978-01-01	29.3
1979-01-01	29.0
1980-01-01	28.6
1981-01-01	28.3
1982-01-01	27.4
1983-01-01	25.9
1984-01-01	25.5
1985-01-01	24.8
1986-01-01	24.1
1987-01-01	23.9
1988-01-01	23.9
1989-01-01	24.2
1990-01-01	23.2
1991-01-01	22.0
1992-01-01	21.7
1993-01-01	21.9
1994-01-01	21.8
1995-01-01	21.4
1996-01-01	21.1
1997-01-01	20.9
1998-01-01	20.6
1999-01-01	20.2
2000-01-01	20.6
2001-01-01	19.8
2002-01-01	20.0
2003-01-01	19.9
2004-01-01	20.3
2005-01-01	20.1
2006-01-01	20.2
2007-01-01	20.2
2008-01-01	20.5
2009-01-01	19.9
2010-01-01	19.7
2011-01-01	19.5
2012-01-01	19.4