| Year | Value | | 2022 | 1.8932455978001 | | 2021 | 1.8340620824079 | | 2020 | 1.7330278204586 | | 2019 | 1.6079813474837 | | 2018 | 1.5371724033626 | | 2017 | 1.3281354335774 | | 2016 | 1.2040556725278 | | 2015 | 1.2965517236469 | | 2014 | 1.4091829734209 | | 2013 | 1.5368976490593 | | 2012 | 1.7979754297027 | | 2011 | 2.1108345776843 | | 2010 | 2.252579345128 | | 2009 | 2.2291625346925 | | 2008 | 2.1788892169985 | | 2007 | 2.141523971563 | | 2006 | 2.0985131417675 | | 2005 | 2.158492155044 | | 2004 | 2.2351492073231 | | 2003 | 2.1893881210377 | | 2002 | 2.5088597595403 | | 2001 | 3.0920791843798 | | 2000 | 3.0755529116678 | | 1999 | 2.8584443558606 | | 1998 | 2.8912382620931 | | 1997 | 2.940607026159 | | 1996 | 3.0461383755575 | | 1995 | 2.9521113159651 | | 1994 | 2.9033779526801 | | 1993 | 2.5585560303495 | | 1992 | 2.6258868108477 | | 1991 | 3.2306433737236 | | 1990 | 3.2974711655988 | | 1989 | 3.3718163587161 | | 1988 | 3.5031041441381 | | 1987 | 3.5465979071245 | | 1986 | 3.5373058551026 | | 1985 | 3.2627387692429 | | 1984 | 3.1593594213046 | | 1983 | 3.6359714697785 | | 1982 | 4.1354283972422 | | 1981 | 4.4231247914962 | | 1980 | 4.0715457864083 | | 1979 | 3.4406393312874 | | 1978 | 3.1763307198306 | | 1977 | 3.1123207931995 | | 1976 | 3.0415116944075 | | 1975 | 2.9461267548392 | | 1974 | 2.8576298860159 | | 1973 | 2.8016444230692 | | 1972 | 2.643489467602 | | 1971 | 2.6429325375234 | | 1970 | 2.7600265356086 | | 1969 | 2.7278332638243 | | 1968 | 2.6997373713752 | | 1967 | 2.6532012024255 | | 1966 | 2.5853963956473 | | 1965 | 2.5179197176561 | | 1964 | 2.4582154523396 | | 1963 | 2.3865038578781 | | 1962 | 2.3124200640011 | | 1961 | 2.3797012269259 |
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