| Year | Value | | 2022 | 3.904926222945 | | 2021 | 3.9427996336459 | | 2020 | 4.2096572714064 | | 2019 | 3.0821491213613 | | 2018 | 3.1301360532557 | | 2017 | 3.1837957941574 | | 2016 | 3.220575013151 | | 2015 | 3.2301533478415 | | 2014 | 3.6778563524448 | | 2013 | 3.8057829012065 | | 2012 | 3.4627500793154 | | 2011 | 3.2974952352035 | | 2010 | 3.3907961010624 | | 2009 | 3.2886518464681 | | 2008 | 3.1835014275572 | | 2007 | 3.046953664066 | | 2006 | 3.109796549169 | | 2005 | 3.2579967820797 | | 2004 | 3.2494675748765 | | 2003 | 3.4942075415127 | | 2002 | 3.4917879924796 | | 2001 | 3.7261358827697 | | 2000 | 2.2109622069717 | | 1999 | 2.9004570340699 | | 1998 | 3.4563343607373 | | 1997 | 4.192659406343 | | 1996 | 3.9490929265744 | | 1995 | 4.3474104939692 | | 1994 | 4.4938193888062 | | 1993 | 4.6555716874048 | | 1992 | 4.2980482787728 | | 1991 | 4.1428429374037 | | 1990 | 4.1488250407024 | | 1989 | 4.3929174133616 | | 1988 | 4.1285158177532 | | 1987 | 4.3693147945153 | | 1986 | 4.3044595576783 | | 1985 | 5.390435226587 | | 1984 | 4.0181908796412 | | 1983 | 4.5056585503621 | | 1982 | 6.2208682734642 | | 1981 | 6.3845939722145 | | 1980 | 5.9400728843299 | | 1979 | 5.6334622823985 | | 1978 | 5.8368087035358 | | 1977 | 6.6197749196141 | | 1976 | 6.2204340404779 | | 1975 | 4.6029128881561 | | 1974 | 3.1458333333333 | | 1973 | 3.061797752809 | | 1972 | 2.8439153439153 | | 1971 | 2.6286248830683 | | 1970 | 2.3160061760165 | | 1969 | 2.390216787104 | | 1968 | 3.004572175049 | | 1967 | 2.6176470588235 | | 1966 | 2.6775700934579 | | 1965 | 2.4293313069909 | | 1964 | 2.835868694956 | | 1963 | 3.1956155143339 | | 1962 | 2.5659133709981 | | 1961 | 2.7024336283186 | | 1960 | 2.3157315731573 |
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