Year	Percent
1970-01-01	10.4
1971-01-01	10.3
1972-01-01	13.6
1973-01-01	13.2
1974-01-01	12.0
1975-01-01	13.2
1976-01-01	15.2
1977-01-01	14.9
1978-01-01	15.4
1979-01-01	16.0
1980-01-01	15.8
1981-01-01	17.3
1982-01-01	19.4
1983-01-01	20.9
1984-01-01	21.6
1985-01-01	22.2
1986-01-01	25.5
1987-01-01	25.8
1988-01-01	24.8
1989-01-01	23.9
1990-01-01	21.3
1991-01-01	21.2
1992-01-01	22.8
1993-01-01	27.3
1994-01-01	28.9
1995-01-01	30.1
1996-01-01	30.1
1997-01-01	29.9
1998-01-01	29.4
1999-01-01	28.2
2000-01-01	26.5
2001-01-01	23.4
2002-01-01	22.3
2003-01-01	24.0
2004-01-01	23.8
2005-01-01	24.2
2006-01-01	21.9
2007-01-01	20.6
2008-01-01	21.5
2009-01-01	25.7
2010-01-01	28.2
2011-01-01	29.4
2012-01-01	35.6