| Year | Value | | 2022 | 2.3909141336678 | | 2021 | 2.4443172194913 | | 2020 | 2.5122501211065 | | 2019 | 2.5376470778001 | | 2018 | 2.5334424422177 | | 2017 | 2.5634938127181 | | 2016 | 2.5895000412874 | | 2015 | 2.5492767189136 | | 2014 | 2.4829320081876 | | 2013 | 2.4423161412979 | | 2012 | 2.4487529981251 | | 2011 | 2.4713111081572 | | 2010 | 2.4582784635231 | | 2009 | 2.4397622703886 | | 2008 | 2.4081942100414 | | 2007 | 2.2933557238869 | | 2006 | 2.0629142789739 | | 2005 | 1.9786677950531 | | 2004 | 2.1292394180662 | | 2003 | 2.2417481340823 | | 2002 | 1.5516855599137 | | 2001 | 1.2959967802549 | | 2000 | 1.9631363846711 | | 1999 | 2.255888510117 | | 1998 | 1.8912553722662 | | 1997 | 2.0571614606842 | | 1996 | 2.8351374367897 | | 1995 | 2.8002993895226 | | 1994 | 3.0196849121354 | | 1993 | 3.0827848266898 | | 1992 | 3.2195246882664 | | 1991 | 4.034339549321 | | 1990 | 3.4907238703605 | | 1989 | 2.6252068838079 | | 1988 | 2.6514805724235 | | 1987 | 2.6000768222303 | | 1986 | 2.4944342271058 | | 1985 | 2.3703327052464 | | 1984 | 2.2428118226341 | | 1983 | 2.1342615644315 | | 1982 | 2.0180877460392 | | 1981 | 1.8888541646015 | | 1980 | 1.773463045397 | | 1979 | 1.656837993555 | | 1978 | 1.5637161134128 | | 1977 | 1.5258811283952 | | 1976 | 1.5300162235594 | | 1975 | 1.5581289314847 | | 1974 | 1.6037780650384 | | 1973 | 1.6557377528326 | | 1972 | 1.709901954362 | | 1971 | 1.7772510875967 | | 1970 | 1.8447938242918 | | 1969 | 1.8962088761808 | | 1968 | 1.9005813153808 | | 1967 | 1.8711801280334 | | 1966 | 1.8477039127486 | | 1965 | 1.8257375632499 | | 1964 | 1.8057658114 | | 1963 | 1.7855875342389 | | 1962 | 1.7641614413662 | | 1961 | 1.7424858932631 |
|