| Year | Value | | 2021 | 15.423288607771 | | 2020 | 15.350207623077 | | 2019 | 14.911721714918 | | 2018 | 14.692478760839 | | 2017 | 14.757206698603 | | 2016 | 14.675575238701 | | 2015 | 14.740105748185 | | 2014 | 14.329609857163 | | 2013 | 14.393774961724 | | 2012 | 14.478402741999 | | 2011 | 14.114313276257 | | 2010 | 13.886519846968 | | 2009 | 12.603071593787 | | 2008 | 13.021679474109 | | 2007 | 13.514464553895 | | 2006 | 13.002020689227 | | 2005 | 12.347617011792 | | 2004 | 11.617830298645 | | 2003 | 10.899371138127 | | 2002 | 10.180911977608 | | 2001 | 10.138525006486 | | 2000 | 10.099792084598 | | 1999 | 10.060328352864 | | 1998 | 10.021230026053 | | 1997 | 9.9828625090895 | | 1996 | 9.9433987773551 | | 1995 | 9.9043004505443 | | 1994 | 9.8652021237334 | | 1993 | 9.8257383919991 | | 1992 | 9.7873708750352 | | 1991 | 9.7490033580712 | | 1990 | 9.7106358411073 | | 1989 | 9.6722683241434 | | 1988 | 9.6346316170264 | | 1987 | 9.5900522163636 | | 1986 | 9.5440111960068 | | 1985 | 9.4979701756501 | | 1984 | 9.4519291552934 | | 1983 | 9.4058881349367 | | 1982 | 9.35984711458 | | 1981 | 9.3138060942233 | | 1980 | 9.2677650738666 | | 1979 | 9.2217240535099 | | 1978 | 9.1756830331532 | | 1977 | 9.1296420127965 | | 1976 | 9.0836009924398 | | 1975 | 9.0375599720831 | | 1974 | 8.9915189517264 | | 1973 | 8.9454779313697 | | 1972 | 8.8994369110129 | | 1971 | 8.8533958906562 | | 1970 | 8.8073548702995 | | 1969 | 8.7613138499428 | | 1968 | 8.7152728295861 | | 1967 | 8.1635113951525 | | 1966 | 7.895669586252 | | 1965 | 7.4765501390366 | | 1964 | 7.3080984693188 | | 1963 | 7.1619364999324 | | 1962 | 7.0084664320767 | | 1961 | 6.7954353616961 |
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