Year	Percent
1973-01-01	27.1
1974-01-01	27.2
1975-01-01	27.5
1976-01-01	28.0
1977-01-01	28.6
1978-01-01	29.2
1979-01-01	30.1
1980-01-01	31.5
1981-01-01	33.0
1982-01-01	34.1
1983-01-01	34.0
1984-01-01	34.8
1985-01-01	34.9
1986-01-01	36.7
1987-01-01	37.9
1988-01-01	38.6
1989-01-01	38.7
1990-01-01	39.5
1991-01-01	39.9
1992-01-01	40.2
1993-01-01	41.0
1994-01-01	41.3
1995-01-01	41.5
1996-01-01	41.8
1997-01-01	42.3
1998-01-01	42.4
1999-01-01	42.9
2000-01-01	43.2
2001-01-01	43.6
2002-01-01	43.9
2003-01-01	44.2
2004-01-01	44.4
2005-01-01	44.9
2006-01-01	45.2
2007-01-01	45.5
2008-01-01	45.7
2009-01-01	45.9
2010-01-01	45.9
2011-01-01	46.2
2012-01-01	46.2