| Year | Value | | 2024 | 17.623929650999 | | 2023 | 18.221930384729 | | 2022 | 17.180109409997 | | 2021 | 17.47526981565 | | 2020 | 17.485624185348 | | 2019 | 16.529978062518 | | 2018 | 17.143298471119 | | 2017 | 17.702534997644 | | 2016 | 18.328007886394 | | 2015 | 18.141924416602 | | 2014 | 17.948896970364 | | 2013 | 18.398470171466 | | 2012 | 18.105140843804 | | 2011 | 18.550815028655 | | 2010 | 18.814982327029 | | 2009 | 18.049456496565 | | 2008 | 17.441498302293 | | 1998 | 14.663417834777 | | 1997 | 13.711315626327 | | 1996 | 11.016517696464 | | 1995 | 10.083278093791 | | 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 |
|