Year	Percent
1963-01-01	27.74
1964-01-01	27.51
1965-01-01	28.26
1966-01-01	28.28
1967-01-01	28.62
1968-01-01	29.38
1969-01-01	29.43
1970-01-01	29.83
1971-01-01	30.25
1972-01-01	30.98
1973-01-01	32.09
1974-01-01	32.92
1975-01-01	33.14
1976-01-01	34.62
1977-01-01	35.19
1978-01-01	36.28
1979-01-01	36.24
1980-01-01	35.89
1981-01-01	36.21
1982-01-01	36.56
1983-01-01	36.34
1984-01-01	35.71
1985-01-01	36.69
1986-01-01	37.62
1987-01-01	39.29
1988-01-01	40.13
1989-01-01	41.37
1990-01-01	42.19
1991-01-01	43.07
1992-01-01	43.45
1993-01-01	43.52
1994-01-01	44.46
1995-01-01	45.27
1996-01-01	45.81
1997-01-01	46.16
1998-01-01	43.08
1999-01-01	43.53
2000-01-01	45.00
2001-01-01	45.55
2002-01-01	46.55
2003-01-01	46.26
2004-01-01	46.96
2005-01-01	47.48
2006-01-01	47.86
2007-01-01	48.22
2008-01-01	48.17
2009-01-01	47.79
2010-01-01	48.23
2011-01-01	48.70