| Year | Value | | 2021 | 1.8512774750706 | | 2020 | 1.8838141725658 | | 2019 | 1.9246739397244 | | 2018 | 1.9747568085651 | | 2017 | 1.9994993138394 | | 2016 | 2.0154403514806 | | 2015 | 2.0683239455933 | | 2014 | 2.0913898706352 | | 2013 | 2.1092984439683 | | 2012 | 2.1355742638709 | | 2011 | 2.1693767192076 | | 2010 | 2.188430283701 | | 2009 | 2.2119784819764 | | 2008 | 2.2339945398015 | | 2007 | 2.2693487277874 | | 2006 | 2.3123949213837 | | 2005 | 2.3578077410434 | | 2004 | 2.3961456950174 | | 2003 | 2.4336580296043 | | 2002 | 2.4971740594008 | | 2001 | 2.5619767515043 | | 2000 | 2.6289604780164 | | 1999 | 2.6951303819827 | | 1998 | 2.7543889533285 | | 1997 | 2.8156490534425 | | 1996 | 2.8803761379823 | | 1995 | 2.9537640597485 | | 1994 | 3.0157786980047 | | 1993 | 3.0811228411263 | | 1992 | 3.1405104955891 | | 1991 | 3.2184239430767 | | 1990 | 3.2985772647935 | | 1989 | 3.380271100872 | | 1988 | 3.462188358854 | | 1987 | 3.5595646248759 | | 1986 | 3.6555527561858 | | 1985 | 3.7539916727413 | | 1984 | 3.852639398361 | | 1983 | 3.9531690763141 | | 1982 | 4.0580029582447 | | 1981 | 4.156564533615 | | 1980 | 4.2601250541589 | | 1979 | 4.3715759219422 | | 1978 | 4.4828456072499 | | 1977 | 4.5805170226617 | | 1976 | 4.6796413906975 | | 1975 | 4.7794913853684 | | 1974 | 4.8883959630937 | | 1973 | 5.0114843171958 | | 1972 | 5.1115777431728 | | 1971 | 5.2116875067586 | | 1970 | 5.312778715415 | | 1969 | 5.4068200676941 | | 1968 | 5.4979522164317 | | 1967 | 5.6061998366321 | | 1966 | 5.7063576231503 | | 1965 | 5.7932078098294 | | 1964 | 5.8715814031528 | | 1963 | 5.9245130857214 | | 1962 | 5.9480886435836 | | 1961 | 5.9569879187469 | | 1960 | 5.9583331481809 |
|