Year	Percent
1970-01-01	38.8
1971-01-01	39.0
1972-01-01	38.8
1973-01-01	38.4
1974-01-01	38.4
1975-01-01	38.3
1976-01-01	37.5
1977-01-01	37.6
1978-01-01	37.4
1979-01-01	37.0
1980-01-01	37.0
1981-01-01	36.5
1982-01-01	36.1
1983-01-01	35.1
1984-01-01	33.4
1985-01-01	32.6
1986-01-01	32.0
1987-01-01	31.5
1988-01-01	31.5
1989-01-01	31.5
1990-01-01	31.4
1991-01-01	31.4
1992-01-01	31.3
1993-01-01	32.5
1994-01-01	32.5
1995-01-01	32.1
1996-01-01	31.6
1997-01-01	31.4
1998-01-01	31.4
1999-01-01	31.1
2000-01-01	30.6
2001-01-01	30.4
2002-01-01	30.3
2003-01-01	30.3
2004-01-01	30.3
2005-01-01	30.4
2006-01-01	29.8
2007-01-01	29.9
2008-01-01	28.9
2009-01-01	28.3
2010-01-01	27.6
2011-01-01	27.3
2012-01-01	26.5