| Year | Value | | 2022 | 3.0469492441799 | | 2021 | 3.3911516360636 | | 2020 | 2.9904758893039 | | 2019 | 2.1897400209959 | | 2018 | 2.3597904319652 | | 2017 | 3.64194893786 | | 2016 | 4.0892970831663 | | 2015 | 4.3013602643704 | | 2014 | 3.7895343792659 | | 2013 | 3.9996365529948 | | 2012 | 3.8543514102702 | | 2005 | 1.3546011650725 | | 2004 | 1.6572270314549 | | 2003 | 1.6704622750803 | | 2002 | 1.1275081612208 | | 2001 | 1.562561491297 | | 2000 | 2.0011992218027 | | 1999 | 1.7313833578484 | | 1998 | 2.0089012110861 | | 1997 | 2.3137223770535 | | 1996 | 3.49342786434 | | 1995 | 3.6848323794625 | | 1994 | 3.5411211276424 | | 1993 | 3.5231085615326 | | 1992 | 3.3545179333988 | | 1991 | 3.1710045930986 | | 1990 | 3.3958905101322 | | 1989 | 2.9591067331911 | | 1988 | 2.1408522749629 | | 1987 | 1.9724009432589 | | 1986 | 2.8791421020533 | | 1985 | 2.9782635874904 | | 1984 | 2.9236711009945 | | 1983 | 3.0703691066375 | | 1982 | 3.2182606652283 | | 1981 | 3.4568203549523 | | 1980 | 3.422129555285 | | 1979 | 3.5575807318937 | | 1978 | 3.9213836477987 | | 1977 | 4.039773110946 | | 1976 | 3.7937069311263 | | 1975 | 3.7749712484559 | | 1974 | 4.0210874508993 | | 1973 | 5.1139455782313 | | 1972 | 5.4516403919898 | | 1971 | 5.6512253991831 | | 1970 | 5.6019450033534 | | 1969 | 5.3104288499025 | | 1968 | 5.023197175996 | | 1967 | 5.2017985226421 | | 1966 | 6.1255794096121 | | 1965 | 6.6956208207683 | | 1964 | 6.0223456471196 | | 1963 | 6.8471406048445 | | 1962 | 5.6907114624506 | | 1961 | 6.1234604986482 |
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