| Year | Value | | 2024 | 18,516 | | 2023 | 18,751 | | 2022 | 18,927 | | 2021 | 19,133 | | 2020 | 19,366 | | 2019 | 19,586 | | 2018 | 19,822 | | 2017 | 19,997 | | 2016 | 20,015 | | 2015 | 19,999 | | 2014 | 20,063 | | 2013 | 20,124 | | 2012 | 20,106 | | 2011 | 19,963 | | 2010 | 19,757 | | 2009 | 19,623 | | 2008 | 19,705 | | 2007 | 20,047 | | 2006 | 20,469 | | 2005 | 20,729 | | 2004 | 20,736 | | 2003 | 20,613 | | 2002 | 20,524 | | 2001 | 20,625 | | 2000 | 20,904 | | 1999 | 21,387 | | 1998 | 21,909 | | 1997 | 22,461 | | 1996 | 23,021 | | 1995 | 23,561 | | 1994 | 24,061 | | 1993 | 24,530 | | 1992 | 25,004 | | 1991 | 25,454 | | 1990 | 25,761 | | 1989 | 25,983 | | 1988 | 26,228 | | 1987 | 26,440 | | 1986 | 26,628 | | 1985 | 26,921 | | 1984 | 27,223 | | 1983 | 27,345 | | 1982 | 27,352 | | 1981 | 27,292 | | 1980 | 26,993 | | 1979 | 26,624 | | 1978 | 26,492 | | 1977 | 26,513 | | 1976 | 26,582 | | 1975 | 26,682 |
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