| Year | Value | | 2024 | 20,208 | | 2023 | 19,887 | | 2022 | 19,582 | | 2021 | 19,432 | | 2020 | 19,341 | | 2019 | 19,225 | | 2018 | 19,168 | | 2017 | 19,200 | | 2016 | 19,276 | | 2015 | 19,449 | | 2014 | 19,671 | | 2013 | 19,906 | | 2012 | 20,221 | | 2011 | 20,509 | | 2010 | 20,666 | | 2009 | 20,827 | | 2008 | 21,033 | | 2007 | 21,137 | | 2006 | 21,173 | | 2005 | 21,264 | | 2004 | 21,363 | | 2003 | 21,414 | | 2002 | 21,344 | | 2001 | 21,081 | | 2000 | 20,671 | | 1999 | 20,230 | | 1998 | 19,592 | | 1997 | 18,832 | | 1996 | 17,876 | | 1995 | 16,640 | | 1994 | 15,373 | | 1993 | 14,350 | | 1992 | 13,734 | | 1991 | 13,678 | | 1990 | 14,160 | | 1989 | 15,053 | | 1988 | 16,105 | | 1987 | 17,016 | | 1986 | 17,602 | | 1985 | 17,789 | | 1984 | 17,626 | | 1983 | 17,255 | | 1982 | 16,820 | | 1981 | 16,423 | | 1980 | 16,130 | | 1979 | 15,924 | | 1978 | 15,766 | | 1977 | 15,572 | | 1976 | 15,152 | | 1975 | 14,434 |
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