Year	Percent
1971-01-01	41.8
1972-01-01	41.1
1973-01-01	40.8
1974-01-01	40.5
1975-01-01	38.9
1976-01-01	38.0
1977-01-01	37.9
1978-01-01	37.6
1979-01-01	37.2
1980-01-01	36.1
1981-01-01	34.2
1982-01-01	33.0
1983-01-01	31.9
1984-01-01	33.2
1985-01-01	32.8
1986-01-01	32.2
1987-01-01	31.2
1988-01-01	31.3
1989-01-01	31.1
1990-01-01	30.7
1991-01-01	31.0
1992-01-01	28.6
1993-01-01	28.1
1994-01-01	26.5
1995-01-01	26.2
1996-01-01	26.3
1997-01-01	24.3
1998-01-01	24.1
1999-01-01	23.3
2000-01-01	22.7
2001-01-01	22.2
2002-01-01	21.7
2003-01-01	20.9
2004-01-01	20.4
2005-01-01	20.2
2006-01-01	20.1
2007-01-01	20.2
2008-01-01	19.5
2009-01-01	18.2
2010-01-01	17.9
2011-01-01	17.7
2012-01-01	17.6