Year	Percent
1960-01-01	34.82
1961-01-01	34.65
1962-01-01	34.97
1963-01-01	35.12
1964-01-01	35.69
1965-01-01	36.33
1966-01-01	37.12
1967-01-01	37.60
1968-01-01	37.78
1969-01-01	38.41
1970-01-01	38.33
1971-01-01	38.13
1972-01-01	38.83
1973-01-01	40.31
1974-01-01	41.45
1975-01-01	41.60
1976-01-01	41.88
1977-01-01	42.11
1978-01-01	42.96
1979-01-01	44.38
1980-01-01	45.10
1981-01-01	45.84
1982-01-01	43.86
1983-01-01	43.75
1984-01-01	44.42
1985-01-01	45.41
1986-01-01	46.30
1987-01-01	46.93
1988-01-01	47.72
1989-01-01	47.93
1990-01-01	47.54
1991-01-01	46.13
1992-01-01	45.14
1993-01-01	44.85
1994-01-01	45.28
1995-01-01	45.61
1996-01-01	45.53
1997-01-01	46.04
1998-01-01	46.78
1999-01-01	47.57
2000-01-01	48.29
2001-01-01	48.36
2002-01-01	48.99
2003-01-01	49.70
2004-01-01	50.04
2005-01-01	50.20
2006-01-01	50.57
2007-01-01	51.23
2008-01-01	51.48
2009-01-01	50.05
2010-01-01	50.14
2011-01-01	50.39