Year	Percent
1960-01-01	47.07
1961-01-01	47.09
1962-01-01	46.71
1963-01-01	46.37
1964-01-01	45.94
1965-01-01	45.69
1966-01-01	45.14
1967-01-01	43.57
1968-01-01	43.45
1969-01-01	43.71
1970-01-01	43.84
1971-01-01	43.57
1972-01-01	43.55
1973-01-01	43.86
1974-01-01	43.39
1975-01-01	42.45
1976-01-01	42.48
1977-01-01	42.67
1978-01-01	43.14
1979-01-01	43.95
1980-01-01	44.54
1981-01-01	44.51
1982-01-01	44.20
1983-01-01	43.95
1984-01-01	44.51
1985-01-01	45.24
1986-01-01	46.08
1987-01-01	46.71
1988-01-01	47.09
1989-01-01	47.50
1990-01-01	48.07
1991-01-01	48.40
1992-01-01	47.38
1993-01-01	46.43
1994-01-01	46.26
1995-01-01	46.29
1996-01-01	46.12
1997-01-01	45.97
1998-01-01	46.51
1999-01-01	47.17
2000-01-01	47.92
2001-01-01	47.95
2002-01-01	47.59
2003-01-01	47.16
2004-01-01	47.31
2005-01-01	47.26
2006-01-01	47.58
2007-01-01	48.45
2008-01-01	49.13
2009-01-01	49.30
2010-01-01	49.60
2011-01-01	50.25