Year	Percent
1970-01-01	27.4
1971-01-01	27.4
1972-01-01	27.3
1973-01-01	27.8
1974-01-01	27.6
1975-01-01	26.1
1976-01-01	25.8
1977-01-01	25.3
1978-01-01	24.8
1979-01-01	24.6
1980-01-01	25.0
1981-01-01	25.1
1982-01-01	24.7
1983-01-01	24.8
1984-01-01	25.2
1985-01-01	25.3
1986-01-01	24.9
1987-01-01	24.4
1988-01-01	24.5
1989-01-01	24.5
1990-01-01	24.3
1991-01-01	24.6
1992-01-01	24.6
1993-01-01	23.9
1994-01-01	23.4
1995-01-01	22.7
1996-01-01	22.5
1997-01-01	22.2
1998-01-01	21.4
1999-01-01	21.0
2000-01-01	20.7
2001-01-01	20.2
2002-01-01	19.1
2003-01-01	18.8
2004-01-01	18.3
2005-01-01	18.1
2006-01-01	18.3
2007-01-01	18.3
2008-01-01	18.1
2009-01-01	17.3
2010-01-01	16.9
2011-01-01	16.8
2012-01-01	16.6