Year	Percent
1970-01-01	29.8
1971-01-01	29.4
1972-01-01	29.2
1973-01-01	29.5
1974-01-01	29.5
1975-01-01	28.1
1976-01-01	27.9
1977-01-01	27.5
1978-01-01	27.3
1979-01-01	27.5
1980-01-01	27.3
1981-01-01	26.9
1982-01-01	25.0
1983-01-01	24.2
1984-01-01	24.3
1985-01-01	24.3
1986-01-01	24.2
1987-01-01	24.1
1988-01-01	24.3
1989-01-01	24.3
1990-01-01	23.6
1991-01-01	22.0
1992-01-01	21.3
1993-01-01	20.7
1994-01-01	20.9
1995-01-01	21.2
1996-01-01	21.1
1997-01-01	21.5
1998-01-01	21.6
1999-01-01	21.7
2000-01-01	21.8
2001-01-01	21.7
2002-01-01	21.8
2003-01-01	21.6
2004-01-01	21.7
2005-01-01	21.4
2006-01-01	20.9
2007-01-01	20.4
2008-01-01	20.4
2009-01-01	19.1
2010-01-01	19.0
2011-01-01	19.1
2012-01-01	19.2