| Year | Value | | 2022 | 2.4721474880828 | | 2021 | 2.5287931800092 | | 2020 | 2.5621532497484 | | 2019 | 2.5826867664182 | | 2018 | 2.6413763453754 | | 2017 | 2.7234853485753 | | 2016 | 2.8040457459926 | | 2015 | 2.8868193843501 | | 2014 | 2.9743344848207 | | 2013 | 3.0512058713028 | | 2012 | 3.0945314000658 | | 2011 | 3.0970434234005 | | 2010 | 3.1011602408817 | | 2009 | 3.1136485108558 | | 2008 | 3.1049859716024 | | 2007 | 3.0834589747555 | | 2006 | 3.021054494209 | | 2005 | 2.9424052038481 | | 2004 | 2.8926516016186 | | 2003 | 2.8516930045262 | | 2002 | 2.8502045287752 | | 2001 | 2.8737097402755 | | 2000 | 2.8964233429194 | | 1999 | 2.928982999032 | | 1998 | 2.9320618973737 | | 1997 | 2.9224690005803 | | 1996 | 2.9285476920743 | | 1995 | 2.9636525031154 | | 1994 | 3.165384519307 | | 1993 | 3.5863306908949 | | 1992 | 3.946433020325 | | 1991 | 4.0416921599285 | | 1990 | 4.0080881419041 | | 1989 | 3.9682463464067 | | 1988 | 3.9185785882658 | | 1987 | 3.8544255644007 | | 1986 | 3.7853428555553 | | 1985 | 3.7082801647886 | | 1984 | 3.6088467752958 | | 1983 | 3.4906203004461 | | 1982 | 3.3655081710425 | | 1981 | 3.3203363238815 | | 1980 | 3.3150549880979 | | 1979 | 3.2428146370082 | | 1978 | 3.1763366102838 | | 1977 | 3.1221671247947 | | 1976 | 3.0505183889358 | | 1975 | 2.98164266813 | | 1974 | 2.9712385031106 | | 1973 | 2.9763866487457 | | 1972 | 2.9391998743357 | | 1971 | 2.9052001819757 | | 1970 | 2.8867612067519 | | 1969 | 2.870352865497 | | 1968 | 2.8487636330441 | | 1967 | 2.796547532686 | | 1966 | 2.7605789288144 | | 1965 | 2.769034832616 | | 1964 | 2.7716333729541 | | 1963 | 2.7703430677088 | | 1962 | 2.7630729181957 | | 1961 | 2.7522250446545 |
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