Year	Percent
1970-01-01	25.0
1971-01-01	24.9
1972-01-01	24.1
1973-01-01	24.5
1974-01-01	25.1
1975-01-01	25.3
1976-01-01	25.8
1977-01-01	26.9
1978-01-01	26.8
1979-01-01	27.3
1980-01-01	27.9
1981-01-01	28.0
1982-01-01	27.6
1983-01-01	27.6
1984-01-01	27.6
1985-01-01	27.8
1986-01-01	28.1
1987-01-01	28.2
1988-01-01	28.3
1989-01-01	28.6
1990-01-01	29.2
1991-01-01	29.7
1992-01-01	29.7
1993-01-01	30.7
1994-01-01	30.3
1995-01-01	30.2
1996-01-01	30.5
1997-01-01	30.7
1998-01-01	31.2
1999-01-01	31.9
2000-01-01	32.7
2001-01-01	33.8
2002-01-01	34.3
2003-01-01	34.2
2004-01-01	34.2
2005-01-01	34.1
2006-01-01	34.7
2007-01-01	34.9
2008-01-01	35.3
2009-01-01	34.7
2010-01-01	34.5
2011-01-01	34.7
2012-01-01	35.0