| Year | Value | | 2021 | 0.081 | | 2020 | 0.083 | | 2019 | 0.085 | | 2018 | 0.084 | | 2017 | 0.076 | | 2013 | 0.058 | | 2012 | 0.056 | | 2011 | 0.053 | | 2010 | 0.05 | | 2009 | 0.046 | | 2006 | 0.026 | | 2004 | 0.026 | | 2000 | 0.024 | | 1990 | 0.012 | | 1989 | 0.028 | | 1985 | 0.021 | | 1982 | 0.026 | | 1981 | 0.027 | | 1980 | 0.025 | | 1970 | 0.053 | | 1965 | 0.056 | | 1960 | 0.043 |
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