| Year | Value | | 2022 | 8.2380324069719 | | 2021 | 2.8152885443583 | | 2020 | 0.73285164517586 | | 2019 | 1.7407126787018 | | 2018 | 1.9324681228636 | | 2017 | 1.8226433605906 | | 2016 | 0.44325815756578 | | 2015 | 0.33870569541017 | | 2014 | 0.62451731108657 | | 2013 | 1.4515652639838 | | 2012 | 2.5294552091577 | | 2011 | 3.3742975943252 | | 2010 | 1.807352305661 | | 2009 | 1.1045771117539 | | 2008 | 4.1140702203382 | | 2007 | 2.4847043881287 | | 2006 | 2.6001537646035 | | 2005 | 2.4695989874028 | | 2004 | 2.2579640963823 | | 2003 | 2.1842835824152 | | 2002 | 2.4773606791094 | | 2001 | 3.1890826810605 | | 2000 | 3.0638034489953 | | 1999 | 2.1726031887621 | | 1998 | 2.1057748113534 | | 1997 | 2.269003059125 | | 1996 | 3.0000907625161 | | 1995 | 4.3517786132837 | | 1994 | 3.502625774134 | | 1993 | 4.5979032100656 | | 1992 | 5.0569779673173 | | 1991 | 5.9342134433592 | | 1990 | 6.7218195222866 | | 1989 | 6.2598313795698 | | 1988 | 5.8248986843944 | | 1987 | 4.7472849038979 | | 1986 | 5.2916617007109 | | 1985 | 6.734693877551 | | 1984 | 7.6738026224732 | | 1983 | 8.8730215236321 | | 1982 | 11.340996168582 | | 1981 | 13.642960812772 | | 1980 | 13.634450606601 | | 1979 | 10.128892275369 | | 1978 | 8.6184321950599 | | 1977 | 11.18202145957 | | 1976 | 11.636752055887 | | 1975 | 14.156872627288 | | 1974 | 15.548520032603 | | 1973 | 9.1434711287436 | | 1972 | 6.5623609025654 | | 1971 | 6.3953488372093 | | 1970 | 5.7311448429353 | | 1969 | 3.4879192439519 | | 1968 | 4.0938914310684 | | 1967 | 3.5799522673031 | | 1966 | 4.1743476185299 | | 1965 | 4.18722443998 | | 1964 | 3.4448984822277 | | 1963 | 2.9669597758771 | | 1962 | 4.2637271214908 | | 1961 | 2.1579731098196 | | 1960 | 1.9457493519864 |
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