| Year | Value | | 2021 | 39.970742355345 | | 2020 | 39.973290478132 | | 2019 | 39.907030906503 | | 2018 | 39.969737532716 | | 2017 | 39.984452025987 | | 2016 | 39.937813931025 | | 2015 | 39.872574517607 | | 2014 | 39.834421496246 | | 2013 | 39.838650168133 | | 2012 | 39.789966964691 | | 2011 | 40.17641904211 | | 2010 | 40.160302621321 | | 2009 | 40.122130989796 | | 2008 | 40.111559647209 | | 2007 | 40.172168974511 | | 2006 | 40.11976585503 | | 2005 | 40.048477976051 | | 2004 | 39.991252656536 | | 2003 | 39.941565760561 | | 2002 | 39.752996808344 | | 2001 | 39.679751445678 | | 2000 | 39.656940780143 | | 1999 | 39.622189599315 | | 1998 | 39.096353462028 | | 1997 | 38.488462616458 | | 1996 | 37.977641035685 | | 1995 | 37.380825917569 | | 1994 | 36.750653887898 | | 1993 | 36.096181217096 | | 1992 | 35.321082698799 | | 1991 | 35.159977169156 | | 1990 | 35.10945338437 | | 1989 | 34.69778408719 | | 1988 | 34.186977382433 | | 1987 | 33.582998852948 | | 1986 | 33.026539276494 | | 1985 | 32.442176641349 | | 1984 | 31.942269614811 | | 1983 | 31.528747460495 | | 1982 | 31.480302813573 | | 1981 | 31.454314324944 | | 1980 | 31.758867614827 | | 1979 | 31.67849387547 | | 1978 | 31.666077822498 | | 1977 | 31.621997161058 | | 1976 | 31.599736427032 | | 1975 | 31.529648175343 | | 1974 | 31.550806892834 | | 1973 | 31.496808082571 | | 1972 | 31.499948095174 | | 1971 | 31.38658766064 | | 1970 | 31.254493000048 | | 1969 | 31.190429293688 | | 1968 | 31.133565407538 | | 1967 | 31.103223308081 | | 1966 | 31.050400137355 | | 1965 | 31.042465641613 | | 1964 | 31.018000946409 | | 1963 | 31.007054280987 | | 1962 | 30.982663053336 | | 1961 | 30.981414104933 |
|