| Year | Value | | 2022 | 26.58880294268 | | 2021 | 25.990539517163 | | 2020 | 23.821061481929 | | 2019 | 24.546150625052 | | 2018 | 26.128324739923 | | 2017 | 28.335987606646 | | 2016 | 28.385001920665 | | 2015 | 28.399147642694 | | 2014 | 29.366566951356 | | 2013 | 29.419840114682 | | 2012 | 30.915349488272 | | 2011 | 34.081802985601 | | 2010 | 33.466468298852 | | 2009 | 33.363441626768 | | 2008 | 38.517708011943 | | 2007 | 38.766842577551 | | 2006 | 38.796474514843 | | 2005 | 36.815754950786 | | 2004 | 35.137496624814 | | 2003 | 34.852651461784 | | 2002 | 32.735084665708 | | 2001 | 32.251779033919 | | 2000 | 35.917379580865 | | 1999 | 38.305980768975 | | 1998 | 39.849158995267 | | 1997 | 37.02054492127 | | 1996 | 37.040723480078 | | 1995 | 33.893255168761 | | 1994 | 35.091416129265 | | 1993 | 34.713172352114 | | 1992 | 31.696475031448 | | 1991 | 29.278262014621 | | 1990 | 30.382680696333 | | 1989 | 30.68049000399 | | 1988 | 31.655299264374 | | 1987 | 31.048899271385 | | 1986 | 25.620862110649 | | 1985 | 25.626694207327 | | 1984 | 28.649905709646 | | 1983 | 26.365623757885 | | 1982 | 23.923704842601 | | 1981 | 25.244302501147 | | 1980 | 29.318301194212 | | 1979 | 33.351283822003 | | 1978 | 27.590666714578 | | 1977 | 27.247990126849 | | 1976 | 28.08260637341 | | 1975 | 20.274843062012 | | 1974 | 23.95660157097 |
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