| Year | Value | | 2019 | 6.8430348171586 | | 2018 | 9.6137991749842 | | 2017 | 9.0032215051581 | | 2016 | 8.3971243146525 | | 2015 | 10.698827839532 | | 2014 | 13.012712292942 | | 2013 | 11.323499721515 | | 2012 | 15.865481226489 | | 2005 | 3.867951231274 | | 2004 | 7.2745570574614 | | 2003 | 2.6980283638879 | | 2002 | 4.7039906186703 | | 2001 | 7.8670062676355 | | 2000 | 7.2444438575409 | | 1999 | 8.9146515407835 | | 1998 | 9.5140592731188 | | 1997 | 17.34204616723 | | 1996 | 25.601938960998 | | 1995 | 20.68097509274 | | 1994 | 21.018854783344 | | 1993 | 24.28973937544 | | 1992 | 23.234896123537 | | 1991 | 27.328244274809 | | 1990 | 23.287671232877 | | 1989 | 27.235083756823 | | 1988 | 29.976649153532 | | 1987 | 23.462154127601 | | 1986 | 23.268571428571 | | 1985 | 26.17914322804 | | 1984 | 25.059769615301 | | 1983 | 26.618861364624 | | 1982 | 30.986527596697 | | 1981 | 28.61169837914 | | 1980 | 24.737267582862 | | 1979 | 24.778270509978 | | 1978 | 21.834747093936 | | 1977 | 19.050694220213 | | 1976 | 18.095238095238 | | 1975 | 25.430107526882 | | 1974 | 20.570264765784 | | 1973 | 17.54068716094 |
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