| Year | Value | | 2022 | 2.2739634085232 | | 2021 | 2.2939948980702 | | 2020 | 2.3892691812242 | | 2019 | 1.9963330121774 | | 2018 | 2.1312823097718 | | 2017 | 2.121098456946 | | 2016 | 2.1454878007674 | | 2015 | 2.1555724036388 | | 2014 | 2.1970951924262 | | 2013 | 1.9965972895897 | | 2012 | 2.1149746661229 | | 2009 | 3.1353241702564 | | 2008 | 3.059333957503 | | 2006 | 2.6936472166972 | | 2005 | 3.051555776233 | | 2004 | 3.8381201116665 | | 2003 | 3.9952757165313 | | 2002 | 2.7393835098468 | | 2001 | 4.002384422489 | | 2000 | 2.9280054454635 | | 1999 | 2.273637888532 | | 1998 | 1.8624409667562 | | 1997 | 2.9672600873691 | | 1996 | 2.6263412071369 | | 1995 | 2.260154633562 | | 1994 | 2.2408619078073 | | 1993 | 2.4124230183432 | | 1992 | 2.6890663988324 | | 1991 | 3.6005124491728 | | 1990 | 3.827926490575 | | 1989 | 3.9611369407608 | | 1988 | 4.4897506002526 | | 1987 | 4.6695869656359 | | 1986 | 5.0367629467775 | | 1985 | 5.9458951718956 |
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