Year	Percent
1970-01-01	26.4
1971-01-01	26.3
1972-01-01	25.6
1973-01-01	26.1
1974-01-01	26.5
1975-01-01	26.8
1976-01-01	27.6
1977-01-01	28.9
1978-01-01	28.8
1979-01-01	29.5
1980-01-01	30.1
1981-01-01	30.4
1982-01-01	30.3
1983-01-01	30.5
1984-01-01	30.5
1985-01-01	30.7
1986-01-01	31.9
1987-01-01	32.2
1988-01-01	32.3
1989-01-01	32.6
1990-01-01	32.8
1991-01-01	33.2
1992-01-01	33.3
1993-01-01	35.7
1994-01-01	35.5
1995-01-01	35.7
1996-01-01	36.1
1997-01-01	36.3
1998-01-01	36.9
1999-01-01	37.4
2000-01-01	37.9
2001-01-01	38.4
2002-01-01	38.7
2003-01-01	38.6
2004-01-01	38.3
2005-01-01	37.9
2006-01-01	38.1
2007-01-01	37.9
2008-01-01	38.6
2009-01-01	38.2
2010-01-01	38.2
2011-01-01	38.4
2012-01-01	39.7