Year	Percent
1971-01-01	33.6
1972-01-01	32.7
1973-01-01	32.1
1974-01-01	32.2
1975-01-01	30.8
1976-01-01	30.0
1977-01-01	30.2
1978-01-01	29.9
1979-01-01	29.3
1980-01-01	28.1
1981-01-01	26.3
1982-01-01	25.3
1983-01-01	24.1
1984-01-01	24.7
1985-01-01	24.6
1986-01-01	24.4
1987-01-01	23.0
1988-01-01	23.3
1989-01-01	22.5
1990-01-01	22.1
1991-01-01	22.6
1992-01-01	21.1
1993-01-01	20.9
1994-01-01	18.9
1995-01-01	18.8
1996-01-01	19.1
1997-01-01	16.6
1998-01-01	16.3
1999-01-01	15.6
2000-01-01	14.8
2001-01-01	14.2
2002-01-01	13.7
2003-01-01	12.6
2004-01-01	11.8
2005-01-01	11.5
2006-01-01	11.2
2007-01-01	11.1
2008-01-01	10.4
2009-01-01	9.7
2010-01-01	9.8
2011-01-01	9.8
2012-01-01	9.8