| Year | Value | | 2020 | 15.077557074564 | | 2019 | 14.609555234921 | | 2018 | 14.252151355235 | | 2017 | 13.451371705053 | | 2016 | 14.414827416291 | | 2015 | 13.29255364252 | | 2014 | 12.946371782808 | | 1992 | 9.048258735629 | | 1991 | 9.2600639607865 | | 1990 | 8.7838062703278 | | 1989 | 10.826929269086 | | 1980 | 10.467635930985 | | 1979 | 11.70219398273 | | 1978 | 9.719924820342 | | 1977 | 10.901857372713 | | 1976 | 9.9062553551299 | | 1975 | 7.5213188029337 | | 1974 | 10.181310485636 | | 1973 | 8.6311478947192 |
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