| Year | Value | | 2024 | 20.246119402388 | | 2023 | 20.526506425195 | | 2022 | 20.800316784209 | | 2021 | 21.067599258194 | | 2020 | 21.322039397176 | | 2019 | 21.644015667008 | | 2018 | 21.972877607551 | | 2017 | 22.279307540146 | | 2016 | 22.559491264851 | | 2015 | 22.809614581725 | | 2014 | 23.025863290823 | | 2013 | 23.204423192205 | | 2012 | 23.341480085927 | | 2011 | 23.433219772046 | | 2010 | 23.475555858529 | | 2009 | 23.48560699597 | | 2008 | 23.490957172897 | | 2007 | 23.495267376524 | | 2006 | 23.501754431634 | | 2005 | 23.525137561973 | | 2004 | 23.672560258098 | | 2003 | 23.953829659449 | | 2002 | 24.315924717101 | | 2001 | 24.709244243201 | | 2000 | 25.079688602974 | | 1999 | 25.409525947796 | | 1998 | 25.728353528258 | | 1997 | 26.048691748016 | | 1996 | 26.373007733637 | | 1995 | 26.703768611688 | | 1994 | 27.043441508738 | | 1993 | 27.394493551352 | | 1992 | 27.759391866099 | | 1991 | 28.140603579545 | | 1990 | 28.540992280008 | | 1989 | 28.962070209075 | | 1988 | 29.406552807651 | | 1987 | 29.873238273342 | | 1986 | 30.361227573953 | | 1985 | 30.86962167729 | | 1984 | 31.397521551157 | | 1983 | 31.944028163358 | | 1982 | 32.5082424817 | | 1981 | 33.089265473986 | | 1980 | 33.685899683956 | | 1979 | 34.30823304801 | | 1978 | 34.965563887757 | | 1977 | 35.65385512423 | | 1976 | 36.368708367277 |
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