| Year | Value | | 2021 | 8.2265565458915 | | 2020 | 8.1572547201686 | | 2019 | 7.9976927181853 | | 2018 | 7.9707496066012 | | 2017 | 7.9318588937118 | | 2016 | 7.9689004824401 | | 2015 | 7.9891449049086 | | 2014 | 7.8893596499002 | | 2013 | 7.8960851115064 | | 2012 | 7.8829135637031 | | 2011 | 7.796564686157 | | 2010 | 7.7434558825578 | | 2009 | 7.5138122162804 | | 2008 | 7.5509403676755 | | 2007 | 7.6075268427667 | | 2006 | 7.4744349152066 | | 2005 | 7.3377493040248 | | 2004 | 7.1926083549488 | | 2003 | 7.0860380517444 | | 2002 | 6.9018068931693 | | 2001 | 6.8581473644084 | | 2000 | 6.8099720175819 | | 1999 | 6.7646439556361 | | 1998 | 6.7016559282767 | | 1997 | 6.6221883174224 | | 1996 | 6.5474482650406 | | 1995 | 6.5021169170299 | | 1994 | 6.501433071313 | | 1993 | 6.5356580359662 | | 1992 | 6.5756030389681 | | 1991 | 6.6050928310823 | | 1990 | 6.6230089934893 | | 1989 | 6.6274040404209 | | 1988 | 6.6335740759597 | | 1987 | 6.6622793095217 | | 1986 | 6.6590335351485 | | 1985 | 6.6975372373189 | | 1984 | 6.6015412235441 | | 1983 | 6.531040975657 | | 1982 | 6.4292195923745 | | 1981 | 6.349098759229 | | 1980 | 6.2703193598447 | | 1979 | 6.0935861002933 | | 1978 | 5.9350650599353 | | 1977 | 5.7603910106283 | | 1976 | 5.5793180850762 | | 1975 | 5.3931983271775 | | 1974 | 5.2949387714903 | | 1973 | 5.2194590647017 | | 1972 | 5.1282493564997 | | 1971 | 5.0074504702309 | | 1970 | 4.9350601657465 | | 1969 | 4.9413477171737 | | 1968 | 4.9053473120996 | | 1967 | 4.7793737153642 | | 1966 | 4.7022538367974 | | 1965 | 4.6173440715067 | | 1964 | 4.5658751770808 | | 1963 | 4.492594599828 | | 1962 | 4.4206494317278 | | 1961 | 4.3556595196311 |
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