Year	Percent
1960-01-01	40.45
1961-01-01	39.51
1962-01-01	39.73
1963-01-01	40.09
1964-01-01	40.66
1965-01-01	41.15
1966-01-01	42.18
1967-01-01	42.47
1968-01-01	42.79
1969-01-01	42.95
1970-01-01	43.81
1971-01-01	42.86
1972-01-01	42.79
1973-01-01	43.36
1974-01-01	43.16
1975-01-01	42.54
1976-01-01	42.52
1977-01-01	42.73
1978-01-01	42.31
1979-01-01	42.58
1980-01-01	43.25
1981-01-01	43.47
1982-01-01	42.71
1983-01-01	41.39
1984-01-01	42.13
1985-01-01	42.86
1986-01-01	43.97
1987-01-01	44.25
1988-01-01	45.17
1989-01-01	46.32
1990-01-01	46.45
1991-01-01	44.80
1992-01-01	44.04
1993-01-01	43.82
1994-01-01	44.70
1995-01-01	45.89
1996-01-01	45.86
1997-01-01	45.83
1998-01-01	46.36
1999-01-01	46.58
2000-01-01	47.20
2001-01-01	47.07
2002-01-01	47.43
2003-01-01	47.93
2004-01-01	48.26
2005-01-01	49.27
2006-01-01	49.80
2007-01-01	50.57
2008-01-01	51.09
2009-01-01	50.53
2010-01-01	51.21
2011-01-01	51.46