Year	Percent
1970-01-01	54.8
1971-01-01	54.6
1972-01-01	54.3
1973-01-01	54.6
1974-01-01	54.5
1975-01-01	53.7
1976-01-01	53.7
1977-01-01	53.7
1978-01-01	53.5
1979-01-01	53.2
1980-01-01	52.8
1981-01-01	52.1
1982-01-01	53.7
1983-01-01	53.2
1984-01-01	52.3
1985-01-01	51.9
1986-01-01	51.9
1987-01-01	51.5
1988-01-01	51.2
1989-01-01	51.5
1990-01-01	51.3
1991-01-01	50.8
1992-01-01	50.4
1993-01-01	49.7
1994-01-01	49.3
1995-01-01	49.6
1996-01-01	49.6
1997-01-01	49.2
1998-01-01	49.3
1999-01-01	49.7
2000-01-01	50.6
2001-01-01	50.9
2002-01-01	51.0
2003-01-01	50.7
2004-01-01	50.4
2005-01-01	50.3
2006-01-01	50.3
2007-01-01	50.9
2008-01-01	51.3
2009-01-01	50.6
2010-01-01	50.5
2011-01-01	50.4
2012-01-01	50.3