| Year | Value | | 2022 | 1.3737153491236 | | 2021 | 1.4028475099569 | | 2020 | 1.4936395787633 | | 2019 | 1.5762397522314 | | 2018 | 1.6177285434623 | | 2017 | 1.6450140128278 | | 2016 | 1.6755092583657 | | 2015 | 1.7212477394283 | | 2014 | 1.7680638456514 | | 2013 | 1.8126713188604 | | 2012 | 1.8493389437415 | | 2011 | 1.8978915683908 | | 2010 | 1.9883070131748 | | 2009 | 2.0423481553295 | | 2008 | 2.0240016086677 | | 2007 | 2.1317951955323 | | 2006 | 1.9175606391233 | | 2005 | 1.5144289622304 | | 2004 | 1.4724684793765 | | 2003 | 2.1663732516708 | | 2002 | 3.0413653925247 | | 2001 | 3.1122435083174 | | 2000 | 3.1016497493543 | | 1999 | 3.4580488052713 | | 1998 | 3.9828968096487 | | 1997 | 3.5261780302918 | | 1996 | 2.359110808772 | | 1995 | 0.98029986313597 | | 1994 | 1.9836919445826 | | 1993 | -1.9387724219635 | | 1992 | 0.85067596789085 | | 1991 | 8.5227392101097 | | 1990 | 10.219241030327 | | 1989 | 7.7306598086043 | | 1988 | 6.4359271116689 | | 1987 | 6.387601261095 | | 1986 | 5.7172278058232 | | 1985 | 4.7406575876898 | | 1984 | 3.8490806581558 | | 1983 | 3.4910494991279 | | 1982 | 3.961886624198 | | 1981 | 5.1691831527109 | | 1980 | 6.7859457846104 | | 1979 | 8.4108721739555 | | 1978 | 9.5431225111516 | | 1977 | 9.8668864059953 | | 1976 | 9.4716384813557 | | 1975 | 8.6024243191188 | | 1974 | 7.63125203529 | | 1973 | 7.0632834701365 | | 1972 | 6.8950075952983 | | 1971 | 6.5970788575733 | | 1970 | 6.428228401295 | | 1969 | 5.872635818479 | | 1968 | 5.2255131950762 | | 1967 | 5.334213684776 | | 1966 | 5.4581927163992 | | 1965 | 5.5059083331007 | | 1964 | 5.2600415569322 | | 1963 | 4.7683988043669 | | 1962 | 4.2244976224445 | | 1961 | 3.8016106844422 |
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