| Year | Value | | 2021 | 40.651616285484 | | 2020 | 40.597145244437 | | 2019 | 40.515767919361 | | 2018 | 40.444243865184 | | 2017 | 40.266297696308 | | 2016 | 40.157790632459 | | 2015 | 40.044465963219 | | 2014 | 39.897267493883 | | 2013 | 39.779079261217 | | 2012 | 39.55014003656 | | 2011 | 39.669556113984 | | 2010 | 39.499586430992 | | 2009 | 39.289775751072 | | 2008 | 39.054486070091 | | 2007 | 38.94337502439 | | 2006 | 38.748053389697 | | 2005 | 38.592560294472 | | 2004 | 38.159211467985 | | 2003 | 38.369250153051 | | 2002 | 38.103398777443 | | 2001 | 37.987661239138 | | 2000 | 37.808950614612 | | 1999 | 37.667016200066 | | 1998 | 37.471268090305 | | 1997 | 37.37720263526 | | 1996 | 37.179043720075 | | 1995 | 36.980340568772 | | 1994 | 36.759426611123 | | 1993 | 36.534554260119 | | 1992 | 37.175365413112 | | 1991 | 37.010036844174 | | 1990 | 36.902312137821 | | 1989 | 36.837772859079 | | 1988 | 36.701801582337 | | 1987 | 36.48447635625 | | 1986 | 36.383942386827 | | 1985 | 36.148341637561 | | 1984 | 35.957645020493 | | 1983 | 35.89659834201 | | 1982 | 35.836241430562 | | 1981 | 35.795881283181 | | 1980 | 35.713097610406 | | 1979 | 35.670965602469 | | 1978 | 35.635667928286 | | 1977 | 35.556681110409 | | 1976 | 35.491869222547 | | 1975 | 35.442867433419 | | 1974 | 35.357048750621 | | 1973 | 35.3364835039 | | 1972 | 35.311617466387 | | 1971 | 35.249958603009 | | 1970 | 35.130589473415 | | 1969 | 35.111048979766 | | 1968 | 35.052984352266 | | 1967 | 34.971582503077 | | 1966 | 34.935387029091 | | 1965 | 34.901418968889 | | 1964 | 34.842443126659 | | 1963 | 34.781897970173 | | 1962 | 34.695535063011 | | 1961 | 34.638786634566 |
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