Year	Percent
1970-01-01	39.5
1971-01-01	37.4
1972-01-01	36.9
1973-01-01	36.7
1974-01-01	36.5
1975-01-01	35.7
1976-01-01	35.2
1977-01-01	35.1
1978-01-01	34.8
1979-01-01	34.5
1980-01-01	34.0
1981-01-01	33.4
1982-01-01	32.8
1983-01-01	32.2
1984-01-01	32.1
1985-01-01	32.3
1986-01-01	32.3
1987-01-01	32.0
1988-01-01	31.6
1989-01-01	31.6
1990-01-01	31.6
1991-01-01	30.7
1992-01-01	29.5
1993-01-01	28.1
1994-01-01	26.6
1995-01-01	25.2
1996-01-01	24.3
1997-01-01	24.0
1998-01-01	24.1
1999-01-01	23.8
2000-01-01	23.9
2001-01-01	23.7
2002-01-01	23.6
2003-01-01	23.3
2004-01-01	23.1
2005-01-01	22.2
2006-01-01	22.0
2007-01-01	22.2
2008-01-01	22.2
2009-01-01	20.5
2010-01-01	20.1
2011-01-01	20.0
2012-01-01	19.8