| Year | Value | | 2021 | 2.3931398419101 | | 2020 | 2.427809079051 | | 2019 | 2.4932420569454 | | 2018 | 2.5328687545377 | | 2017 | 2.6023113958895 | | 2016 | 2.623863503946 | | 2015 | 2.6205733299779 | | 2014 | 2.6555357358104 | | 2013 | 2.6682800144832 | | 2012 | 2.7057696528646 | | 2011 | 2.6905316557392 | | 2010 | 2.7139572852777 | | 2009 | 2.7435028756417 | | 2008 | 2.7567197278873 | | 2007 | 2.7606854295989 | | 2006 | 2.7684675643018 | | 2005 | 2.7877614742559 | | 2004 | 2.8118776939554 | | 2003 | 2.8265079793586 | | 2002 | 2.8594304882438 | | 2001 | 2.8919081534319 | | 2000 | 2.9334909941671 | | 1999 | 2.9320181414005 | | 1998 | 2.9678189627325 | | 1997 | 3.012644649094 | | 1996 | 3.0664965594811 | | 1995 | 3.1276411041391 | | 1994 | 3.1895862852696 | | 1993 | 3.2557324871393 | | 1992 | 3.3363438831241 | | 1991 | 3.4424818320103 | | 1990 | 3.6665695573332 | | 1989 | 3.7267319258144 | | 1988 | 3.7926335574986 | | 1987 | 3.9192570627468 | | 1986 | 3.9619336015338 | | 1985 | 3.980630742897 | | 1984 | 4.0345015631618 | | 1983 | 4.0687045720273 | | 1982 | 4.22996676423 | | 1981 | 4.2141043821443 | | 1980 | 4.2433477235985 | | 1979 | 4.2817179339409 | | 1978 | 4.31528172435 | | 1977 | 4.400233690066 | | 1976 | 4.5615835628346 | | 1975 | 4.6925169927454 | | 1974 | 4.9119089872771 | | 1973 | 5.1168505980338 | | 1972 | 5.2711137194046 | | 1971 | 5.4238243315131 | | 1970 | 5.6120103955187 | | 1969 | 5.6633314192896 | | 1968 | 5.7849204529312 | | 1967 | 5.6164457870536 | | 1966 | 5.7825003392758 | | 1965 | 5.8899472819907 | | 1964 | 5.9282857969334 | | 1963 | 6.1734424727813 | | 1962 | 5.7769339724181 | | 1961 | 5.1366498894862 | | 1960 | 5.3011340018046 |
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