| Year | Value | | 2021 | 8.484168951498 | | 2020 | 7.6925927290741 | | 2019 | 7.2017152738379 | | 2018 | 7.2164948545987 | | 2017 | 7.2336661557941 | | 2016 | 7.2886231787916 | | 2015 | 7.3504392495474 | | 2014 | 7.4202060075126 | | 2013 | 7.5017313399625 | | 2012 | 7.5916066522997 | | 2011 | 7.6813337313441 | | 2010 | 7.7962573428049 | | 2009 | 7.8586489824026 | | 2008 | 7.9818832475254 | | 2007 | 8.0002208667537 | | 2006 | 8.0492730347276 | | 2005 | 8.1135850577939 | | 2004 | 8.2002538341714 | | 2003 | 8.2640748353672 | | 2002 | 8.3553797621194 | | 2001 | 8.4304366850992 | | 2000 | 8.5114628466375 | | 1999 | 8.5876482989104 | | 1998 | 8.7336104074654 | | 1997 | 8.8019477875681 | | 1996 | 8.9444068481516 | | 1995 | 9.0553822622657 | | 1994 | 9.2180788941221 | | 1993 | 9.2432305359781 | | 1992 | 9.2737963646535 | | 1991 | 9.3596931080631 | | 1990 | 9.4034099182781 | | 1989 | 9.4531458753083 | | 1988 | 9.7326613689606 | | 1987 | 9.7989457328082 | | 1986 | 9.9905383496947 | | 1985 | 10.210480409533 | | 1984 | 10.392329557145 | | 1983 | 10.52640596365 | | 1982 | 10.50375742379 | | 1981 | 10.587253280076 | | 1980 | 10.734273982651 | | 1979 | 10.831286794657 | | 1978 | 11.013402005547 | | 1977 | 11.343339391414 | | 1976 | 11.751427232432 | | 1975 | 11.955822016002 | | 1974 | 12.051873792407 | | 1973 | 12.122259514161 | | 1972 | 12.535009241363 | | 1971 | 13.173245854674 | | 1970 | 12.980139266713 | | 1969 | 13.150698271744 | | 1968 | 13.361793254757 | | 1967 | 13.782500913098 | | 1966 | 14.112663925216 | | 1965 | 14.653349307334 | | 1964 | 14.930705057637 | | 1963 | 14.753921092567 | | 1962 | 14.9542494445 | | 1961 | 16.3385571103 | | 1960 | 19.864795295624 |
|