Year	Percent
1970-01-01	27.7
1971-01-01	28.0
1972-01-01	27.9
1973-01-01	27.9
1974-01-01	28.1
1975-01-01	28.0
1976-01-01	27.7
1977-01-01	27.6
1978-01-01	27.3
1979-01-01	26.9
1980-01-01	26.9
1981-01-01	26.3
1982-01-01	25.8
1983-01-01	25.0
1984-01-01	23.9
1985-01-01	23.2
1986-01-01	22.9
1987-01-01	22.5
1988-01-01	22.6
1989-01-01	22.8
1990-01-01	22.6
1991-01-01	22.1
1992-01-01	22.0
1993-01-01	24.0
1994-01-01	24.3
1995-01-01	24.1
1996-01-01	23.7
1997-01-01	23.5
1998-01-01	23.7
1999-01-01	23.4
2000-01-01	22.9
2001-01-01	22.4
2002-01-01	22.3
2003-01-01	22.1
2004-01-01	21.9
2005-01-01	21.6
2006-01-01	21.2
2007-01-01	21.2
2008-01-01	20.1
2009-01-01	19.5
2010-01-01	18.9
2011-01-01	19.0
2012-01-01	18.6