Year Number 1972 3.1260689729341653 1973 2.733501512566499 1974 2.504607176767297 1975 2.063438619409051 1976 2.1103413279512697 1977 1.952406724711793 1978 2.255656701128094 1979 1.3939723071855057 1980 1.335699386868495 1981 1.5293729832293543 1982 1.6531945013242688 1983 2.338044595194282 1984 2.265396670712992 1985 2.2899476337281133 1986 2.1155829001424413 1987 1.550857212179359 1988 2.1603847422264355 1989 1.8854805121880387 1990 1.709009177087689 1991 1.2172709550201435 1992 1.3687589110806428 1993 1.7849409739760917 1994 2.0784391668886055 1995 2.0235978435564963 1996 2.0538832206788475 1997 2.456147363836026 1998 2.7421018608659593 1999 2.067854723795057 2000 2.5420318821230574 2001 2.833163584334081 2002 2.3041891066816986 2003 2.084150804328105 2004 2.1668747578126126 2005 2.1759510726935085 2006 2.3536742352429587 2007 1.6688651879519847 2008 1.6915533046258011 2009 1.6814665120684886 2010 1.75408171164185 2011 1.7840768779873282 2012 1.9706637480797131 2013 1.9301491271433568 2014 2.112870563281886 2015 2.2453702515345944 2016 1.979739623102716 2017 2.5975465852312696