Year	Percent
1975-01-01	49.31
1976-01-01	48.71
1977-01-01	47.69
1978-01-01	47.11
1979-01-01	48.02
1980-01-01	49.26
1981-01-01	49.67
1982-01-01	49.95
1983-01-01	49.84
1984-01-01	49.88
1985-01-01	49.75
1986-01-01	49.43
1987-01-01	49.56
1988-01-01	49.92
1989-01-01	50.21
1990-01-01	49.75
1991-01-01	46.69
1992-01-01	43.15
1993-01-01	40.38
1994-01-01	39.64
1995-01-01	40.19
1996-01-01	40.62
1997-01-01	41.89
1998-01-01	42.55
1999-01-01	43.50
2000-01-01	44.31
2001-01-01	44.80
2002-01-01	45.11
2003-01-01	45.03
2004-01-01	45.08
2005-01-01	45.54
2006-01-01	46.20
2007-01-01	47.01
2008-01-01	48.00
2009-01-01	46.52
2010-01-01	46.28
2011-01-01	46.59