| Year | Value | | 2022 | 2.9585734522546 | | 2021 | 3.0072872334075 | | 2020 | 3.0138532838535 | | 2019 | 3.02174626028 | | 2018 | 3.1892371709134 | | 2017 | 3.3711508050387 | | 2016 | 3.4768765008259 | | 2015 | 3.3445743048552 | | 2014 | 3.1199435830831 | | 2013 | 3.0247757912925 | | 2012 | 2.9090681260736 | | 2011 | 2.8529261346151 | | 2010 | 2.5882525494075 | | 2009 | 2.5040207533938 | | 2008 | 2.7296238795863 | | 2007 | 2.806371236276 | | 2006 | 2.8064970773452 | | 2005 | 2.7729814698765 | | 2004 | 2.7135921028327 | | 2003 | 2.6610905029697 | | 2002 | 2.6170872742526 | | 2001 | 2.7213753181563 | | 2000 | 2.8368079427147 | | 1999 | 2.6413852540135 | | 1998 | 2.6103683934955 | | 1997 | 2.0504391377027 | | 1996 | 1.8822417391544 | | 1995 | 3.1976583274419 | | 1994 | 3.8901331198659 | | 1993 | 3.1699286451118 | | 1992 | 2.532892413406 | | 1991 | 2.5799312443103 | | 1990 | 2.6412840069592 | | 1989 | 2.6953818421956 | | 1988 | 3.0434625258069 | | 1987 | 3.2693116991171 | | 1986 | 3.288947363612 | | 1985 | 3.2075687039941 | | 1984 | 3.1910335420502 | | 1983 | 3.2197871996265 | | 1982 | 3.0157232321872 | | 1981 | 3.0280055720025 | | 1980 | 3.1533213193445 | | 1979 | 3.3643948511069 | | 1978 | 3.5201860419167 | | 1977 | 3.5911956738787 | | 1976 | 3.5979916235407 | | 1975 | 3.5880863146215 | | 1974 | 3.558194503892 | | 1973 | 3.552319366085 | | 1972 | 3.5104166143902 | | 1971 | 3.4216184181397 | | 1970 | 3.3633457435887 | | 1969 | 3.3029185624637 | | 1968 | 3.1388613786406 | | 1967 | 2.9955440634897 | | 1966 | 2.9760034612476 | | 1965 | 2.9761763303377 | | 1964 | 2.9611079357343 | | 1963 | 2.921218310697 | | 1962 | 2.9238904993223 | | 1961 | 2.899397807579 |
|