Year	Percent
1970-01-01	38.0
1971-01-01	37.1
1972-01-01	36.3
1973-01-01	36.3
1974-01-01	36.4
1975-01-01	35.8
1976-01-01	34.8
1977-01-01	33.7
1978-01-01	32.4
1979-01-01	31.9
1980-01-01	31.5
1981-01-01	30.6
1982-01-01	29.4
1983-01-01	29.1
1984-01-01	29.0
1985-01-01	29.1
1986-01-01	29.2
1987-01-01	28.7
1988-01-01	28.3
1989-01-01	28.4
1990-01-01	28.2
1991-01-01	27.2
1992-01-01	25.6
1993-01-01	24.5
1994-01-01	24.0
1995-01-01	25.0
1996-01-01	25.2
1997-01-01	25.0
1998-01-01	24.9
1999-01-01	24.2
2000-01-01	23.6
2001-01-01	23.1
2002-01-01	22.5
2003-01-01	22.0
2004-01-01	21.9
2005-01-01	21.1
2006-01-01	21.1
2007-01-01	20.8
2008-01-01	20.8
2009-01-01	19.2
2010-01-01	19.0
2011-01-01	19.0
2012-01-01	18.7