| Year | Value | | 2022 | 1.233485548496 | | 2021 | 1.8988970406366 | | 2020 | 2.4315679126999 | | 2019 | 2.6936502400112 | | 2018 | 2.5236954094523 | | 2017 | 2.0500658175222 | | 2016 | 2.0503246050325 | | 2015 | 1.8128482927445 | | 2014 | 1.8719665511272 | | 2013 | 1.9239593325934 | | 2012 | 2.0094477989455 | | 2011 | 2.0275065915051 | | 2010 | 2.2717142190295 | | 2009 | 2.4893117421156 | | 2008 | 2.199273923442 | | 2007 | 2.2179293211353 | | 2006 | 2.3595222796582 | | 2005 | 2.4093705059219 | | 2004 | 2.6979837169098 | | 2003 | 3.2960471726413 | | 2002 | 3.7959268811379 | | 2001 | 3.6034570748273 | | 2000 | 3.6610946697956 | | 1999 | 3.8890173740367 | | 1998 | 3.1845009085201 | | 1997 | 4.1036722305683 | | 1996 | 4.1396969426189 | | 1995 | 3.901649734749 | | 1994 | 4.051359336992 | | 1993 | 3.9214172544983 | | 1992 | 3.8705665002104 | | 1991 | 3.7543642480797 | | 1990 | 3.5277056937872 | | 1989 | 3.1487265209167 | | 1988 | 2.932208636434 | | 1987 | 3.3285852504838 | | 1986 | 3.6379924825209 | | 1985 | 3.5310431587826 | | 1984 | 3.5970938149502 | | 1983 | 3.9376052054601 | | 1982 | 4.2981513121269 | | 1981 | 3.8245318699791 | | 1980 | 3.9003132966797 | | 1979 | 3.3572585580073 | | 1978 | 4.1873064036918 | | 1977 | 4.7136230237622 | | 1976 | 4.9430272108843 | | 1975 | 5.1194539249147 | | 1974 | 3.1930213795885 | | 1973 | 3.3494505494505 | | 1972 | 3.4455205811138 | | 1971 | 3.499793814433 | | 1970 | 3.2564885496183 | | 1969 | 3.0793378995434 | | 1968 | 3.2755555555556 | | 1967 | 3.2595744680851 | | 1966 | 3.1439811172305 | | 1965 | 3.5544186046512 | | 1964 | 3.4224652087475 | | 1963 | 3.387339055794 | | 1962 | 3.7110834371108 | | 1961 | 3.7645429362881 | | 1960 | 3.5315712187959 |
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