Year	Percent
1971-01-01	42.3
1972-01-01	42.6
1973-01-01	43.6
1974-01-01	44.2
1975-01-01	44.2
1976-01-01	44.0
1977-01-01	44.0
1978-01-01	44.4
1979-01-01	45.0
1980-01-01	45.1
1981-01-01	44.3
1982-01-01	43.5
1983-01-01	43.3
1984-01-01	44.4
1985-01-01	45.2
1986-01-01	45.8
1987-01-01	46.9
1988-01-01	48.4
1989-01-01	49.9
1990-01-01	50.3
1991-01-01	49.6
1992-01-01	49.1
1993-01-01	49.1
1994-01-01	49.3
1995-01-01	49.8
1996-01-01	50.4
1997-01-01	51.0
1998-01-01	51.4
1999-01-01	52.0
2000-01-01	52.5
2001-01-01	52.7
2002-01-01	53.0
2003-01-01	53.1
2004-01-01	53.4
2005-01-01	53.7
2006-01-01	53.8
2007-01-01	53.6
2008-01-01	53.8
2009-01-01	53.2
2010-01-01	52.9
2011-01-01	52.8
2012-01-01	52.9