Year	Percent
1970-01-01	38.1
1971-01-01	38.2
1972-01-01	38.6
1973-01-01	39.1
1974-01-01	38.7
1975-01-01	37.9
1976-01-01	37.8
1977-01-01	37.7
1978-01-01	37.9
1979-01-01	38.4
1980-01-01	38.8
1981-01-01	38.7
1982-01-01	38.1
1983-01-01	37.3
1984-01-01	37.3
1985-01-01	37.6
1986-01-01	38.1
1987-01-01	38.5
1988-01-01	39.0
1989-01-01	39.7
1990-01-01	40.5
1991-01-01	44.3
1992-01-01	43.4
1993-01-01	42.7
1994-01-01	42.3
1995-01-01	42.7
1996-01-01	42.8
1997-01-01	42.7
1998-01-01	43.5
1999-01-01	44.1
2000-01-01	44.4
2001-01-01	44.8
2002-01-01	44.7
2003-01-01	44.5
2004-01-01	44.3
2005-01-01	44.9
2006-01-01	46.0
2007-01-01	47.1
2008-01-01	47.8
2009-01-01	48.3
2010-01-01	48.9
2011-01-01	50.2
2012-01-01	50.4