Year	Percent
1960-01-01	37.80
1961-01-01	37.19
1962-01-01	37.27
1963-01-01	37.25
1964-01-01	37.54
1965-01-01	37.99
1966-01-01	38.67
1967-01-01	39.16
1968-01-01	39.59
1969-01-01	40.17
1970-01-01	39.92
1971-01-01	39.58
1972-01-01	40.31
1973-01-01	41.24
1974-01-01	41.63
1975-01-01	40.76
1976-01-01	41.69
1977-01-01	42.75
1978-01-01	44.10
1979-01-01	44.84
1980-01-01	44.53
1981-01-01	44.59
1982-01-01	43.80
1983-01-01	43.97
1984-01-01	45.37
1985-01-01	45.87
1986-01-01	46.47
1987-01-01	47.24
1988-01-01	47.83
1989-01-01	48.33
1990-01-01	48.36
1991-01-01	47.27
1992-01-01	46.89
1993-01-01	46.91
1994-01-01	47.36
1995-01-01	47.46
1996-01-01	47.56
1997-01-01	48.03
1998-01-01	48.15
1999-01-01	48.32
2000-01-01	48.98
2001-01-01	48.51
2002-01-01	47.91
2003-01-01	47.94
2004-01-01	48.03
2005-01-01	48.41
2006-01-01	48.84
2007-01-01	48.88
2008-01-01	48.22
2009-01-01	46.04
2010-01-01	45.40
2011-01-01	45.32