Year	Percent
1973-01-01	27.3
1974-01-01	27.1
1975-01-01	27.1
1976-01-01	27.3
1977-01-01	28.2
1978-01-01	28.4
1979-01-01	29.2
1980-01-01	31.0
1981-01-01	31.9
1982-01-01	32.8
1983-01-01	31.9
1984-01-01	32.7
1985-01-01	33.4
1986-01-01	34.7
1987-01-01	35.4
1988-01-01	37.0
1989-01-01	37.7
1990-01-01	39.4
1991-01-01	40.9
1992-01-01	41.9
1993-01-01	42.7
1994-01-01	43.2
1995-01-01	44.3
1996-01-01	45.3
1997-01-01	47.2
1998-01-01	48.6
1999-01-01	50.3
2000-01-01	51.8
2001-01-01	53.3
2002-01-01	54.1
2003-01-01	54.0
2004-01-01	53.8
2005-01-01	54.1
2006-01-01	55.0
2007-01-01	56.6
2008-01-01	57.5
2009-01-01	57.6
2010-01-01	55.7
2011-01-01	55.8
2012-01-01	55.9