Year	Percent
1960-01-01	37.82
1961-01-01	37.74
1962-01-01	37.91
1963-01-01	37.88
1964-01-01	37.88
1965-01-01	37.66
1966-01-01	37.41
1967-01-01	37.03
1968-01-01	36.97
1969-01-01	36.87
1970-01-01	36.11
1971-01-01	36.12
1972-01-01	35.13
1973-01-01	35.06
1974-01-01	34.97
1975-01-01	34.13
1976-01-01	33.31
1977-01-01	33.91
1978-01-01	33.82
1979-01-01	35.04
1980-01-01	34.36
1981-01-01	33.85
1982-01-01	33.39
1983-01-01	32.87
1984-01-01	32.07
1985-01-01	31.85
1986-01-01	31.64
1987-01-01	31.83
1988-01-01	31.99
1989-01-01	32.11
1990-01-01	33.49
1991-01-01	33.17
1992-01-01	33.04
1993-01-01	33.37
1994-01-01	34.29
1995-01-01	35.62
1996-01-01	36.61
1997-01-01	38.25
1998-01-01	41.06
1999-01-01	43.24
2000-01-01	44.58
2001-01-01	45.24
2002-01-01	45.16
2003-01-01	45.27
2004-01-01	45.99
2005-01-01	47.19
2006-01-01	48.08
2007-01-01	48.69
2008-01-01	47.39
2009-01-01	43.24
2010-01-01	41.30
2011-01-01	40.33