| Year | Value | | 2022 | 1.9663688478836 | | 2021 | 1.6819055174741 | | 2020 | 1.7636793921087 | | 2019 | 1.9089826395799 | | 2018 | 1.9266228652024 | | 2017 | 1.938725674411 | | 2016 | 1.9603489624817 | | 2015 | 1.9812889756208 | | 2014 | 1.9195437985959 | | 2013 | 1.8660528947867 | | 2012 | 1.8461513212671 | | 2011 | 1.848077861596 | | 2010 | 1.8267524098785 | | 2009 | 2.3902073380122 | | 2008 | 1.7765791059698 | | 2007 | 1.7761008691798 | | 2006 | 1.7678058735819 | | 2005 | 1.7628976611301 | | 2004 | 1.7676236415499 | | 2003 | 1.8055197588811 | | 2002 | 2.0140583940473 | | 2001 | 2.357117299468 | | 2000 | 2.6423319130576 | | 1999 | 2.5464338865935 | | 1998 | 2.2868786904316 | | 1997 | 2.4983950432933 | | 1996 | 2.6165059425276 | | 1995 | 2.668286956618 | | 1994 | 2.589262848748 | | 1993 | 2.6580917791605 | | 1992 | 3.4554668987316 | | 1991 | 6.0170087832194 | | 1990 | 3.0946030091743 | | 1989 | 1.6964692573576 | | 1988 | 1.6570575502937 | | 1987 | 1.6151712455154 | | 1986 | 1.5471475975358 | | 1985 | 1.7636300778012 | | 1984 | 1.3068917856383 | | 1983 | 1.8191258769363 | | 1982 | 1.8781070374563 | | 1981 | 1.9913856940006 | | 1980 | 2.4009504033329 | | 1979 | 2.5683594734695 | | 1978 | 2.1088005886933 | | 1977 | 2.2391083788495 | | 1976 | 2.2324911157375 | | 1975 | 2.2834715816203 | | 1974 | 2.9754261081793 | | 1973 | 4.0466149766481 | | 1972 | 2.5415554917558 | | 1971 | 3.1443927599636 | | 1970 | 3.3159763468552 | | 1969 | 2.605781238676 | | 1968 | 2.0909197221157 | | 1967 | 4.317752921375 | | 1966 | 2.542509836581 | | 1965 | 3.493805136128 | | 1964 | 3.9560164699833 | | 1963 | 3.6819227445053 | | 1962 | 4.8245175328214 | | 1961 | 3.3024480406323 |
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