Year	Percent
1960-01-01	41.26
1961-01-01	41.31
1962-01-01	41.55
1963-01-01	41.54
1964-01-01	41.71
1965-01-01	41.49
1966-01-01	41.27
1967-01-01	40.68
1968-01-01	40.64
1969-01-01	40.84
1970-01-01	40.96
1971-01-01	40.88
1972-01-01	40.23
1973-01-01	40.18
1974-01-01	40.13
1975-01-01	39.76
1976-01-01	39.74
1977-01-01	39.54
1978-01-01	39.83
1979-01-01	40.40
1980-01-01	40.82
1981-01-01	40.36
1982-01-01	39.57
1983-01-01	39.07
1984-01-01	39.24
1985-01-01	39.79
1986-01-01	40.51
1987-01-01	41.10
1988-01-01	41.57
1989-01-01	42.44
1990-01-01	43.44
1991-01-01	43.92
1992-01-01	44.20
1993-01-01	44.08
1994-01-01	44.12
1995-01-01	44.88
1996-01-01	45.69
1997-01-01	46.86
1998-01-01	47.79
1999-01-01	48.69
2000-01-01	49.43
2001-01-01	50.29
2002-01-01	50.59
2003-01-01	50.06
2004-01-01	49.80
2005-01-01	49.71
2006-01-01	50.54
2007-01-01	51.66
2008-01-01	52.25
2009-01-01	52.00
2010-01-01	50.38
2011-01-01	50.12