| Year | Value | | 2022 | 4.5113914425637 | | 2021 | 4.976052225862 | | 2020 | 5.2753502408277 | | 2019 | 5.0528461529668 | | 2018 | 5.2828285890252 | | 2017 | 5.4300610356479 | | 2016 | 5.4147820542948 | | 2015 | 5.4239703775208 | | 2014 | 5.6429116296547 | | 2013 | 5.4571872142551 | | 2012 | 5.6419727401468 | | 2011 | 5.7696198271599 | | 2010 | 5.9363450285841 | | 2009 | 6.1481119285256 | | 2008 | 6.3918010973703 | | 2007 | 6.367865558416 | | 2006 | 6.0495644548393 | | 2005 | 6.2649126904147 | | 2004 | 6.3580351131766 | | 2003 | 6.486405973476 | | 2002 | 6.5940133400159 | | 2001 | 6.5080631231006 | | 2000 | 6.2971649834627 | | 1999 | 6.5301233016481 | | 1998 | 6.98021666561 | | 1997 | 7.4792678598745 | | 1996 | 7.7296541873387 | | 1995 | 7.9187439064677 | | 1994 | 10.119965835376 | | 1993 | 11.523243990747 | | 1992 | 10.925077527679 | | 1991 | 15.58698291366 | | 1990 | 12.362686187267 | | 1989 | 12.445732654722 | | 1988 | 15.947094557291 | | 1987 | 16.900144948031 | | 1986 | 14.767712882714 | | 1985 | 16.600643527798 | | 1984 | 16.182228915663 | | 1983 | 15.54688867708 | | 1982 | 18.38821008006 | | 1981 | 20.26226861979 | | 1980 | 18.91933663047 | | 1979 | 19.384693189481 | | 1978 | 22.891134975784 | | 1977 | 23.087213342599 | | 1976 | 29.245593098636 | | 1975 | 30.463802786271 | | 1974 | 27.655286442041 | | 1973 | 27.856092653735 | | 1972 | 20.371018842755 | | 1971 | 22.799803477592 | | 1970 | 23.4470563208 | | 1969 | 19.089316987741 | | 1968 | 17.428014721801 | | 1967 | 15.442514474772 | | 1966 | 9.4346733668342 | | 1965 | 8.5691537761601 | | 1964 | 8.9846269619437 | | 1963 | 9.7535741158766 | | 1962 | 9.160791960402 | | 1961 | 7.8650162631008 | | 1960 | 7.4602483119146 |
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