Year	Percent
1971-01-01	40.31
1972-01-01	40.32
1973-01-01	40.90
1974-01-01	40.68
1975-01-01	39.51
1976-01-01	38.63
1977-01-01	37.85
1978-01-01	36.43
1979-01-01	35.35
1980-01-01	34.33
1981-01-01	33.31
1982-01-01	32.85
1983-01-01	32.58
1984-01-01	31.73
1985-01-01	31.28
1986-01-01	31.91
1987-01-01	33.36
1988-01-01	34.45
1989-01-01	35.63
1990-01-01	36.93
1991-01-01	37.29
1992-01-01	36.65
1993-01-01	35.50
1994-01-01	35.23
1995-01-01	35.82
1996-01-01	36.25
1997-01-01	37.21
1998-01-01	38.53
1999-01-01	39.72
2000-01-01	40.76
2001-01-01	41.61
2002-01-01	42.02
2003-01-01	42.65
2004-01-01	43.49
2005-01-01	44.55
2006-01-01	45.62
2007-01-01	46.16
2008-01-01	45.35
2009-01-01	42.01
2010-01-01	40.78
2011-01-01	39.92