Year	Percent
1970-01-01	7.5
1971-01-01	8.9
1972-01-01	9.9
1973-01-01	9.9
1974-01-01	10.9
1975-01-01	13.3
1976-01-01	16.1
1977-01-01	19.0
1978-01-01	20.0
1979-01-01	22.8
1980-01-01	26.5
1981-01-01	30.9
1982-01-01	24.0
1983-01-01	24.8
1984-01-01	29.8
1985-01-01	29.2
1986-01-01	25.7
1987-01-01	23.4
1988-01-01	20.5
1989-01-01	16.4
1990-01-01	17.3
1991-01-01	19.2
1992-01-01	21.3
1993-01-01	23.3
1994-01-01	26.8
1995-01-01	22.4
1996-01-01	25.0
1997-01-01	25.4
1998-01-01	22.9
1999-01-01	24.2
2000-01-01	21.4
2001-01-01	20.2
2002-01-01	21.3
2003-01-01	23.4
2004-01-01	26.5
2005-01-01	26.1
2006-01-01	29.1
2007-01-01	26.1
2008-01-01	23.9
2009-01-01	30.9
2010-01-01	29.7
2011-01-01	29.4
2012-01-01	32.8