Year	Percent
1970-01-01	39.4
1971-01-01	39.1
1972-01-01	38.7
1973-01-01	38.7
1974-01-01	38.1
1975-01-01	37.9
1976-01-01	37.9
1977-01-01	38.7
1978-01-01	39.0
1979-01-01	39.1
1980-01-01	38.9
1981-01-01	38.9
1982-01-01	39.1
1983-01-01	39.6
1984-01-01	39.5
1985-01-01	39.5
1986-01-01	39.6
1987-01-01	39.8
1988-01-01	40.0
1989-01-01	40.3
1990-01-01	40.5
1991-01-01	40.6
1992-01-01	40.6
1993-01-01	40.5
1994-01-01	40.5
1995-01-01	40.5
1996-01-01	40.5
1997-01-01	40.7
1998-01-01	40.7
1999-01-01	40.6
2000-01-01	40.8
2001-01-01	40.9
2002-01-01	40.9
2003-01-01	41.1
2004-01-01	41.3
2005-01-01	41.4
2006-01-01	41.6
2007-01-01	41.6
2008-01-01	41.7
2009-01-01	42.0
2010-01-01	42.2
2011-01-01	42.1
2012-01-01	42.4