| Year | Value | | 2020 | 21.287239602778 | | 2019 | 19.225399421213 | | 2018 | 20.518527337166 | | 2017 | 21.467112103706 | | 2016 | 22.126195099085 | | 2015 | 21.848723771526 | | 2014 | 19.361075587734 | | 2013 | 20.172432829525 | | 2012 | 21.426120985111 | | 2009 | 37.450886439866 | | 1999 | 41.087103308575 | | 1998 | 50.96089008766 | | 1997 | 38.142707240294 | | 1996 | 56.426332288401 | | 1995 | 55.735411670664 | | 1990 | 66.020950280674 |
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