Year	Percent
1970-01-01	12.6
1971-01-01	12.5
1972-01-01	17.5
1973-01-01	16.5
1974-01-01	15.3
1975-01-01	17.2
1976-01-01	20.1
1977-01-01	19.1
1978-01-01	19.8
1979-01-01	21.1
1980-01-01	20.3
1981-01-01	22.2
1982-01-01	25.5
1983-01-01	28.3
1984-01-01	29.5
1985-01-01	30.2
1986-01-01	33.4
1987-01-01	33.2
1988-01-01	31.2
1989-01-01	29.9
1990-01-01	27.3
1991-01-01	26.7
1992-01-01	29.5
1993-01-01	33.4
1994-01-01	35.4
1995-01-01	35.6
1996-01-01	35.7
1997-01-01	35.8
1998-01-01	36.7
1999-01-01	36.1
2000-01-01	33.7
2001-01-01	30.4
2002-01-01	29.2
2003-01-01	31.9
2004-01-01	35.0
2005-01-01	37.1
2006-01-01	33.2
2007-01-01	31.7
2008-01-01	35.0
2009-01-01	40.3
2010-01-01	45.8
2011-01-01	48.7
2012-01-01	56.9