| Year | Value | | 2024 | 20,455 | | 2023 | 21,068 | | 2022 | 21,619 | | 2021 | 22,024 | | 2020 | 22,342 | | 2019 | 22,549 | | 2018 | 22,619 | | 2017 | 22,566 | | 2016 | 22,505 | | 2015 | 22,418 | | 2014 | 22,271 | | 2013 | 22,116 | | 2012 | 21,930 | | 2011 | 21,727 | | 2010 | 21,525 | | 2009 | 21,324 | | 2008 | 21,071 | | 2007 | 20,865 | | 2006 | 20,827 | | 2005 | 20,909 | | 2004 | 21,099 | | 2003 | 21,502 | | 2002 | 21,910 | | 2001 | 22,176 | | 2000 | 22,425 | | 1999 | 22,713 | | 1998 | 22,879 | | 1997 | 22,969 | | 1996 | 22,993 | | 1995 | 22,983 | | 1994 | 22,927 | | 1993 | 22,806 | | 1992 | 22,600 | | 1991 | 22,402 | | 1990 | 22,301 | | 1989 | 22,217 | | 1988 | 21,994 | | 1987 | 21,525 | | 1986 | 20,726 | | 1985 | 19,568 | | 1984 | 18,229 | | 1983 | 17,116 | | 1982 | 16,483 | | 1981 | 16,430 | | 1980 | 17,062 | | 1979 | 18,126 | | 1978 | 19,105 | | 1977 | 19,785 | | 1976 | 20,157 | | 1975 | 20,138 |
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