Year	Percent
1970-01-01	38.4
1971-01-01	38.5
1972-01-01	39.0
1973-01-01	39.5
1974-01-01	39.4
1975-01-01	39.3
1976-01-01	39.3
1977-01-01	39.3
1978-01-01	39.6
1979-01-01	39.9
1980-01-01	40.3
1981-01-01	40.7
1982-01-01	40.7
1983-01-01	40.5
1984-01-01	40.7
1985-01-01	41.2
1986-01-01	41.4
1987-01-01	41.7
1988-01-01	42.3
1989-01-01	42.7
1990-01-01	43.1
1991-01-01	47.7
1992-01-01	47.4
1993-01-01	47.2
1994-01-01	47.1
1995-01-01	47.1
1996-01-01	47.4
1997-01-01	47.8
1998-01-01	48.2
1999-01-01	48.3
2000-01-01	48.4
2001-01-01	48.6
2002-01-01	48.7
2003-01-01	48.8
2004-01-01	49.2
2005-01-01	50.3
2006-01-01	51.2
2007-01-01	51.6
2008-01-01	51.8
2009-01-01	52.1
2010-01-01	52.4
2011-01-01	53.2
2012-01-01	53.2