| Year | Value | | 2023 | 18.535406868493 | | 2022 | 16.092071489332 | | 2021 | 17.863637365908 | | 2020 | 17.663723025617 | | 2019 | 16.524385837799 | | 2018 | 17.270074623977 | | 2017 | 17.718157742726 | | 2016 | 18.265587225563 | | 2015 | 18.180780675559 | | 2014 | 17.945817047853 | | 2013 | 18.363808102917 | | 2012 | 18.107181635709 | | 2011 | 18.570917865519 | | 2010 | 18.859020486632 | | 2009 | 18.073934946411 | | 2008 | 17.502287306223 | | 1998 | 14.668380101208 | | 1997 | 19.050538393081 | | 1996 | 15.199521054209 | | 1995 | 13.976774696349 | | 1994 | 15.201467696539 | | 1993 | 13.342771571313 | | 1992 | 12.968826598181 | | 1991 | 12.495173514629 | | 1990 | 11.55833126834 | | 1989 | 11.249191132125 | | 1988 | 11.027165901203 | | 1987 | 12.141955705088 | | 1986 | 11.728195178869 | | 1985 | 11.345614110071 | | 1984 | 10.80096921867 | | 1983 | 13.938069007177 | | 1981 | 15.268953297051 | | 1980 | 14.338699193209 | | 1979 | 14.59270724596 | | 1978 | 16.511789620077 | | 1977 | 16.661933162927 | | 1976 | 16.265792031098 | | 1975 | 14.722308408315 |
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