| Year | Value | | 2021 | 1.5168461661823 | | 2020 | 1.5945118889614 | | 2019 | 1.7365122866853 | | 2018 | 1.776758852117 | | 2017 | 1.9538583769111 | | 2016 | 1.9347064639234 | | 2015 | 1.8740499461492 | | 2014 | 1.949767460536 | | 2013 | 1.9189360005847 | | 2012 | 1.9841483216574 | | 2011 | 1.8942917578319 | | 2010 | 1.906844508779 | | 2009 | 1.9323913617875 | | 2008 | 1.9285902250058 | | 2007 | 1.9060278138239 | | 2006 | 1.8859431174206 | | 2005 | 1.8734304618472 | | 2004 | 1.8642461542675 | | 2003 | 1.846376075477 | | 2002 | 1.8525762774935 | | 2001 | 1.8590369685882 | | 2000 | 1.9124471722994 | | 1999 | 1.8546895013734 | | 1998 | 1.868762729949 | | 1997 | 1.8939791443467 | | 1996 | 1.9293534811671 | | 1995 | 1.9704533752403 | | 1994 | 2.0186581086823 | | 1993 | 2.0836965561947 | | 1992 | 2.164251646737 | | 1991 | 2.3018605923355 | | 1990 | 2.7391433129486 | | 1989 | 2.7661385328077 | | 1988 | 2.8050212775954 | | 1987 | 2.992561666689 | | 1986 | 2.9927796905857 | | 1985 | 2.9676049905011 | | 1984 | 2.9947364896695 | | 1983 | 2.9956532652719 | | 1982 | 3.3310147248297 | | 1981 | 3.2260248618186 | | 1980 | 3.2200547843236 | | 1979 | 3.2520807681192 | | 1978 | 3.2583739436228 | | 1977 | 3.3815432848043 | | 1976 | 3.703220164488 | | 1975 | 3.9833757604734 | | 1974 | 4.4529196077188 | | 1973 | 4.907980762259 | | 1972 | 5.220684093544 | | 1971 | 5.5454651760571 | | 1970 | 5.9804922801673 | | 1969 | 6.0639036650967 | | 1968 | 6.3254076962221 | | 1967 | 5.8183023363878 | | 1966 | 6.2022331635432 | | 1965 | 6.4379755568407 | | 1964 | 6.4979467726075 | | 1963 | 7.1236286538767 | | 1962 | 6.0737132068858 | | 1961 | 4.4075629398867 | | 1960 | 4.8642290597105 |
|