Year	Percent
1970-01-01	48.7
1971-01-01	47.7
1972-01-01	47.0
1973-01-01	46.8
1974-01-01	45.9
1975-01-01	44.5
1976-01-01	44.1
1977-01-01	43.8
1978-01-01	43.5
1979-01-01	43.3
1980-01-01	42.9
1981-01-01	42.2
1982-01-01	41.3
1983-01-01	40.6
1984-01-01	40.3
1985-01-01	40.1
1986-01-01	39.9
1987-01-01	39.5
1988-01-01	39.0
1989-01-01	38.9
1990-01-01	38.9
1991-01-01	39.8
1992-01-01	38.9
1993-01-01	37.8
1994-01-01	36.6
1995-01-01	35.3
1996-01-01	34.4
1997-01-01	33.9
1998-01-01	33.6
1999-01-01	33.2
2000-01-01	32.8
2001-01-01	32.3
2002-01-01	31.7
2003-01-01	31.0
2004-01-01	30.7
2005-01-01	29.2
2006-01-01	29.0
2007-01-01	29.2
2008-01-01	29.0
2009-01-01	27.5
2010-01-01	27.1
2011-01-01	26.9
2012-01-01	26.9