| Year | Value | | 2021 | 2.3394477991721 | | 2020 | 2.0014431658501 | | 2019 | 2.0736014583571 | | 2018 | 1.9691618244655 | | 2017 | 1.8647221905739 | | 2016 | 2.0546124340131 | | 2015 | 1.8153507272796 | | 2014 | 2.2407048725836 | | 2013 | 2.0773992632259 | | 2012 | 1.933082678212 | | 2011 | 1.7887660931981 | | 2010 | 1.8951046295241 | | 2009 | 1.7887660931981 | | 2008 | 1.72040560556 | | 2007 | 1.6368538984467 | | 2006 | 1.5153241426455 | | 2005 | 1.614067069234 | | 2004 | 1.59507804489 | | 2003 | 1.5760890205461 | | 2002 | 1.5244388743306 | | 2001 | 1.5255782157913 | | 2000 | 1.5338540123808 | | 1999 | 1.5308757738027 | | 1998 | 1.5203869963161 | | 1997 | 1.5099248034636 | | 1996 | 1.5070886027876 | | 1995 | 1.4968975732027 | | 1994 | 1.4866833770081 | | 1993 | 1.484123276746 | | 1992 | 1.4700816528047 | | 1991 | 1.4469636550074 | | 1990 | 1.4431658501386 | | 1989 | 1.4374691428354 | | 1988 | 1.4203790209259 | | 1987 | 1.59507804489 | | 1986 | 1.7659792639854 | | 1985 | 1.7811704834606 | | 1984 | 1.7317990201663 | | 1983 | 1.6957198739129 | | 1982 | 1.6634385325282 | | 1981 | 1.5779879229805 | | 1980 | 1.5191219475143 | | 1979 | 1.59507804489 | | 1978 | 1.5301355816338 | | 1977 | 1.5236793133569 | | 1976 | 1.5172230450799 | | 1975 | 1.4564581671794 | | 1974 | 1.3956932892788 | | 1973 | 1.4114541794843 | | 1972 | 1.4272150696897 | | 1971 | 1.4279746306635 | | 1970 | 1.4279746306635 | | 1969 | 1.435570240401 | | 1968 | 1.435570240401 | | 1967 | 1.435570240401 | | 1966 | 1.4431658501386 | | 1965 | 1.4431658501386 | | 1964 | 1.4431658501386 | | 1963 | 1.4431658501386 | | 1962 | 1.4469636550074 | | 1961 | 1.4469636550074 |
|