Year Percent 1976-01-01 0.455272000000000 1977-01-01 1.835710000000000 1978-01-01 2.168830000000000 1979-01-01 2.887140000000000 1980-01-01 1.372150000000000 1981-01-01 2.108690000000000 1982-01-01 3.300790000000000 1983-01-01 3.292310000000000 1984-01-01 4.345540000000000 1985-01-01 4.135390000000000 1986-01-01 4.170340000000000 1987-01-01 4.715460000000000 1988-01-01 5.029740000000000 1989-01-01 5.439350000000000 1990-01-01 7.603020000000000 1991-01-01 9.048060000000001 1992-01-01 11.943200000000000 1993-01-01 11.911900000000000 1994-01-01 12.653800000000000 1995-01-01 11.924500000000000 1996-01-01 11.306100000000000 1997-01-01 11.734700000000000 1998-01-01 12.237000000000000 1999-01-01 12.282700000000000 2000-01-01 14.970000000000000 2001-01-01 15.667000000000000 2002-01-01 15.418300000000000 2003-01-01 16.017200000000000 2004-01-01 18.674200000000000 2005-01-01 20.605500000000000 2006-01-01 21.767000000000000 2007-01-01 21.802900000000000 2008-01-01 20.874900000000000 2009-01-01 19.329700000000000 2010-01-01 18.819700000000000 2011-01-01 17.964800000000000 2012-01-01 18.301800000000000 2013-01-01 18.048000000000000 2014-01-01 18.420300000000000 2015-01-01 18.532400000000000 2016-01-01 19.232300000000000 2017-01-01 20.374700000000000