| Year | Value | | 2020 | 11.879196873741 | | 2019 | 13.021723912496 | | 2018 | 13.193028747716 | | 2017 | 12.605001762351 | | 2016 | 12.948628583278 | | 2015 | 15.227287056527 | | 2014 | 17.909962151047 | | 2002 | 21.013246964952 | | 2001 | 24.69644969845 | | 2000 | 18.975964995173 | | 1999 | 21.788167463356 | | 1998 | 20.480291147675 | | 1997 | 23.723204700831 | | 1996 | 22.463184540892 | | 1995 | 19.72715385733 | | 1994 | 20.596007449867 | | 1993 | 19.864395636005 | | 1992 | 18.284158181388 | | 1991 | 17.733525626803 | | 1990 | 20.073467264807 |
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