Year Percent 1990 0.48367 1991 0.4608 1992 0.4475 1993 0.43497 1994 0.41144 1995 0.57376 1996 0.56239 1997 0.55808 1998 0.56695 1999 0.52462 2000 0.49822 2001 0.49467 2002 0.48023 2003 0.46983 2004 0.44376 2005 0.43576 2006 0.42669 2007 0.4174 2008 0.40812 2009 0.44524 2010 0.43459 2011 0.41909 2012 0.41297 2013 0.40574 2014 0.39237 2015 0.3883 2016 0.37823 2017 0.35602 2018 0.35011 2019 0.34374 2020 0.37012