| Year | Value | | 2022 | 3.0967526706733 | | 2021 | 3.1660298615302 | | 2020 | 3.2683484045846 | | 2019 | 3.395277791971 | | 2018 | 3.4644569836099 | | 2017 | 3.5509866361995 | | 2016 | 3.5862110068324 | | 2015 | 3.617677517621 | | 2014 | 3.6844289668349 | | 2013 | 3.7355254285757 | | 2012 | 3.7587025219413 | | 2011 | 3.7587963850454 | | 2010 | 3.7347976584385 | | 2009 | 3.6934824764561 | | 2008 | 3.6709092022535 | | 2007 | 3.6395893043959 | | 2006 | 3.5920133134417 | | 2005 | 3.5576589891038 | | 2004 | 3.5063888237041 | | 2003 | 3.4133212124076 | | 2002 | 3.3351316084388 | | 2001 | 3.2852172329909 | | 2000 | 3.2441214667285 | | 1999 | 3.2264478527189 | | 1998 | 3.2790677476155 | | 1997 | 3.3351537815267 | | 1996 | 3.3301324983654 | | 1995 | 3.2896758261493 | | 1994 | 3.1984325378482 | | 1993 | 3.1615466675544 | | 1992 | 3.2488500228073 | | 1991 | 3.3261371742574 | | 1990 | 3.3451436469421 | | 1989 | 3.3713509500874 | | 1988 | 3.3670427765118 | | 1987 | 3.4386737357978 | | 1986 | 3.5658560351414 | | 1985 | 3.6037510091581 | | 1984 | 3.6098724116677 | | 1983 | 3.6171365330338 | | 1982 | 3.5921948846811 | | 1981 | 3.5544222089439 | | 1980 | 3.5039846550809 | | 1979 | 3.4354475219256 | | 1978 | 3.398701247718 | | 1977 | 3.3174486836903 | | 1976 | 3.2740728463574 | | 1975 | 3.3283239225804 | | 1974 | 3.3523597455642 | | 1973 | 3.2993743446626 | | 1972 | 2.9931319408251 | | 1971 | 2.4143391078011 | | 1970 | 1.6945257673912 | | 1969 | 1.0203546491043 | | 1968 | 0.69598473645663 | | 1967 | 0.69669809166994 | | 1966 | 0.87580632261598 | | 1965 | 1.1110406414361 | | 1964 | 1.3017451764171 | | 1963 | 1.4104253086281 | | 1962 | 1.4607383704534 | | 1961 | 1.5583550408936 |
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