Year	Percent
1960-01-01	38.25
1961-01-01	38.41
1962-01-01	38.84
1963-01-01	38.83
1964-01-01	39.01
1965-01-01	38.81
1966-01-01	38.68
1967-01-01	38.30
1968-01-01	38.11
1969-01-01	38.65
1970-01-01	38.64
1971-01-01	38.84
1972-01-01	38.61
1973-01-01	38.83
1974-01-01	39.28
1975-01-01	38.60
1976-01-01	38.33
1977-01-01	38.13
1978-01-01	38.12
1979-01-01	38.44
1980-01-01	38.34
1981-01-01	37.64
1982-01-01	37.16
1983-01-01	36.68
1984-01-01	36.67
1985-01-01	36.83
1986-01-01	37.05
1987-01-01	37.29
1988-01-01	37.89
1989-01-01	38.34
1990-01-01	38.64
1991-01-01	38.62
1992-01-01	38.42
1993-01-01	38.02
1994-01-01	37.74
1995-01-01	38.21
1996-01-01	38.23
1997-01-01	38.40
1998-01-01	38.99
1999-01-01	39.44
2000-01-01	40.13
2001-01-01	40.54
2002-01-01	40.30
2003-01-01	40.09
2004-01-01	40.34
2005-01-01	40.69
2006-01-01	40.88
2007-01-01	41.25
2008-01-01	41.66
2009-01-01	41.25
2010-01-01	41.22
2011-01-01	41.54