Year | Value | 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 | 1974 | 13.170633319887 |
|