| Year | Value | | 2022 | 2.6601608400417 | | 2021 | 2.6724784617948 | | 2020 | 2.8670821953274 | | 2019 | 3.2642485702589 | | 2018 | 2.9838235530317 | | 2017 | 2.2873014469589 | | 2016 | 1.6290247538838 | | 2015 | 2.1887060904562 | | 2014 | 3.3458633833966 | | 2013 | 3.5511077040435 | | 2012 | 3.5299728477603 | | 2011 | 3.5427449811879 | | 2010 | 4.6794776094182 | | 2009 | 5.078443764683 | | 2008 | 4.1120975235659 | | 2007 | 3.6717922033092 | | 2006 | 3.5800308116882 | | 2005 | 3.6997969367081 | | 2004 | 3.7146628169662 | | 2003 | 3.1373615082863 | | 2002 | 2.7941375778703 | | 2001 | 2.4751157586687 | | 2000 | 2.0417212041938 | | 1999 | 2.3715206268295 | | 1998 | 1.8643589927417 | | 1997 | 0.11211057621028 | | 1996 | 0.038370469371051 | | 1995 | 6.0156906921208 | | 1994 | 0.55984783301524 | | 1993 | -3.3258496131708 | | 1992 | 2.615392284182 | | 1991 | 2.0045584740676 | | 1990 | 2.0280888796049 | | 1989 | 1.9136678294928 | | 1988 | 2.1875326070035 | | 1987 | 2.2759604784719 | | 1986 | 1.8756211908153 | | 1985 | 2.160509894961 | | 1984 | 2.4196965692723 | | 1983 | 2.663636300115 | | 1982 | 2.45317104137 | | 1981 | 4.042293827212 | | 1980 | 4.1518714632647 | | 1979 | 2.3977872296876 | | 1978 | 2.8113882997114 | | 1977 | 2.4887867566594 | | 1976 | 2.1545872132634 | | 1975 | 2.4079128463483 | | 1974 | 2.4500214749565 | | 1973 | 0.21662389976503 | | 1972 | 0.080293081025735 | | 1971 | 2.3796925821407 | | 1970 | 2.1209177294212 | | 1969 | 2.1259130126242 | | 1968 | 2.35125644182 | | 1967 | 2.5083757251995 | | 1966 | 2.384454471177 | | 1965 | 2.7609654777045 | | 1964 | 2.8453002560072 | | 1963 | 2.0816338093197 | | 1962 | 2.5048255484112 | | 1961 | 2.4933517580268 |
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