Year	Percent
1973-01-01	35.1
1974-01-01	34.3
1975-01-01	33.2
1976-01-01	32.3
1977-01-01	32.1
1978-01-01	31.8
1979-01-01	31.0
1980-01-01	29.7
1981-01-01	28.4
1982-01-01	27.5
1983-01-01	27.3
1984-01-01	27.3
1985-01-01	26.6
1986-01-01	26.1
1987-01-01	26.3
1988-01-01	25.7
1989-01-01	25.9
1990-01-01	25.8
1991-01-01	25.0
1992-01-01	24.3
1993-01-01	24.0
1994-01-01	22.9
1995-01-01	22.8
1996-01-01	22.3
1997-01-01	22.3
1998-01-01	21.6
1999-01-01	21.6
2000-01-01	21.3
2001-01-01	21.2
2002-01-01	20.4
2003-01-01	19.6
2004-01-01	20.0
2005-01-01	20.1
2006-01-01	19.7
2007-01-01	18.9
2008-01-01	17.9
2009-01-01	17.2
2010-01-01	16.8
2011-01-01	16.3
2012-01-01	16.2