Year	Percent
1970-01-01	28.3
1971-01-01	28.3
1972-01-01	28.1
1973-01-01	28.8
1974-01-01	29.1
1975-01-01	29.4
1976-01-01	30.2
1977-01-01	31.4
1978-01-01	31.5
1979-01-01	32.2
1980-01-01	32.7
1981-01-01	33.0
1982-01-01	33.2
1983-01-01	33.5
1984-01-01	33.7
1985-01-01	34.0
1986-01-01	34.9
1987-01-01	35.3
1988-01-01	35.6
1989-01-01	36.0
1990-01-01	36.3
1991-01-01	36.3
1992-01-01	36.6
1993-01-01	37.9
1994-01-01	38.1
1995-01-01	38.4
1996-01-01	38.8
1997-01-01	39.0
1998-01-01	39.4
1999-01-01	39.8
2000-01-01	40.1
2001-01-01	40.5
2002-01-01	40.6
2003-01-01	40.7
2004-01-01	40.7
2005-01-01	40.5
2006-01-01	40.6
2007-01-01	40.6
2008-01-01	41.0
2009-01-01	41.1
2010-01-01	41.3
2011-01-01	41.6
2012-01-01	42.3