| Year | Value | | 2021 | 2.4901270890182 | | 2020 | 2.4838173424794 | | 2019 | 2.3863376470122 | | 2018 | 2.5587568139628 | | 2017 | 2.5689751833238 | | 2016 | 2.4633897796413 | | 2015 | 2.4751383842337 | | 2014 | 2.7683142050537 | | 2013 | 2.935208992769 | | 2012 | 2.8983047735993 | | 2011 | 2.8912520569327 | | 2010 | 2.7674002878446 | | 2009 | 2.9206093669849 | | 2008 | 3.4558623715464 | | 2004 | 4.3957286121522 | | 2003 | 4.3642370837397 | | 2002 | 4.8009096499472 | | 2001 | 4.7350092490486 | | 1999 | 2.3279221358254 | | 1998 | 1.605551057011 | | 1997 | 2.9836118747494 | | 1996 | 3.7026777469991 | | 1995 | 4.4532984400959 | | 1994 | 6.7144591357984 | | 1993 | 6.1000337952011 | | 1992 | 5.9600133932885 | | 1991 | 5.4555220215868 | | 1990 | 6.6406458714028 | | 1989 | 5.9487217932379 | | 1988 | 5.5609983671565 | | 1987 | 4.9875179263823 | | 1986 | 6.4029880610952 | | 1985 | 3.4176003603401 | | 1984 | 3.4820314039813 | | 1983 | 4.0152825836217 | | 1982 | 4.3645484949833 | | 1981 | 4.5205363920047 | | 1980 | 4.5261669024045 | | 1979 | 4.8799261083744 | | 1978 | 7.2481572481573 | | 1977 | 8.5485307212823 | | 1976 | 9.2518973617636 | | 1975 | 8.3612040133779 | | 1974 | 9.4152046783626 | | 1973 | 14.147909967846 | | 1972 | 13.556338028169 |
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