Year	Percent
1970-01-01	33.1
1971-01-01	31.7
1972-01-01	31.4
1973-01-01	32.0
1974-01-01	31.4
1975-01-01	29.5
1976-01-01	29.6
1977-01-01	29.7
1978-01-01	30.0
1979-01-01	30.2
1980-01-01	29.3
1981-01-01	28.9
1982-01-01	27.2
1983-01-01	25.7
1984-01-01	26.1
1985-01-01	25.8
1986-01-01	25.5
1987-01-01	25.0
1988-01-01	24.8
1989-01-01	24.7
1990-01-01	24.2
1991-01-01	23.4
1992-01-01	22.8
1993-01-01	22.3
1994-01-01	22.3
1995-01-01	22.4
1996-01-01	22.3
1997-01-01	22.4
1998-01-01	22.2
1999-01-01	21.7
2000-01-01	22.0
2001-01-01	21.3
2002-01-01	20.3
2003-01-01	20.0
2004-01-01	20.0
2005-01-01	19.8
2006-01-01	19.9
2007-01-01	19.8
2008-01-01	19.1
2009-01-01	17.6
2010-01-01	17.2
2011-01-01	17.3
2012-01-01	17.3