Year	Percent
1971-01-01	39.4
1972-01-01	39.6
1973-01-01	39.6
1974-01-01	39.4
1975-01-01	38.9
1976-01-01	38.1
1977-01-01	37.8
1978-01-01	37.1
1979-01-01	36.6
1980-01-01	36.1
1981-01-01	35.5
1982-01-01	33.5
1983-01-01	32.8
1984-01-01	32.0
1985-01-01	31.0
1986-01-01	30.2
1987-01-01	29.8
1988-01-01	29.6
1989-01-01	29.4
1990-01-01	29.5
1991-01-01	29.2
1992-01-01	28.3
1993-01-01	27.2
1994-01-01	26.5
1995-01-01	26.1
1996-01-01	25.6
1997-01-01	25.4
1998-01-01	25.1
1999-01-01	24.9
2000-01-01	24.6
2001-01-01	24.6
2002-01-01	24.2
2003-01-01	23.8
2004-01-01	23.5
2005-01-01	23.2
2006-01-01	23.1
2007-01-01	22.8
2008-01-01	22.2
2009-01-01	21.4
2010-01-01	20.9
2011-01-01	20.9
2012-01-01	20.4