Year	Percent
1960-01-01	48.83
1961-01-01	49.00
1962-01-01	48.99
1963-01-01	48.96
1964-01-01	49.29
1965-01-01	49.17
1966-01-01	48.77
1967-01-01	47.88
1968-01-01	48.11
1969-01-01	48.67
1970-01-01	49.17
1971-01-01	48.74
1972-01-01	48.76
1973-01-01	48.85
1974-01-01	49.67
1975-01-01	50.45
1976-01-01	50.45
1977-01-01	50.37
1978-01-01	50.41
1979-01-01	51.03
1980-01-01	51.50
1981-01-01	51.52
1982-01-01	51.39
1983-01-01	51.49
1984-01-01	51.86
1985-01-01	52.32
1986-01-01	52.51
1987-01-01	52.76
1988-01-01	53.24
1989-01-01	53.65
1990-01-01	53.73
1991-01-01	52.57
1992-01-01	49.94
1993-01-01	47.05
1994-01-01	46.27
1995-01-01	46.78
1996-01-01	46.33
1997-01-01	45.70
1998-01-01	46.45
1999-01-01	47.39
2000-01-01	48.48
2001-01-01	49.36
2002-01-01	49.22
2003-01-01	48.76
2004-01-01	48.22
2005-01-01	48.16
2006-01-01	48.70
2007-01-01	49.45
2008-01-01	49.51
2009-01-01	47.91
2010-01-01	48.02
2011-01-01	48.71