| Year | Value | | 2021 | 2.2433948239746 | | 2020 | 2.3199765749938 | | 2019 | 2.4190823704304 | | 2018 | 2.2640234287002 | | 2017 | 2.1063302545618 | | 2016 | 2.6724487497184 | | 2015 | 2.5670195990088 | | 2014 | 2.6751520612751 | | 2013 | 2.6019373732823 | | 2012 | 2.444244199144 | | 2011 | 1.9779229556206 | | 2010 | 2.0004505519261 | | 2009 | 2.2617706690696 | | 2008 | 1.6942429737081 | | 2007 | 1.5899818676337 | | 2006 | 2.1532184950136 | | 2005 | 2.096554850408 | | 2004 | 2.368540344515 | | 2003 | 2.005893019039 | | 2002 | 2.2098821396192 | | 2001 | 2.1758839528559 | | 2000 | 2.1532184950136 | | 1999 | 2.1532184950136 | | 1998 | 2.1532184950136 | | 1997 | 2.1192203082502 | | 1996 | 2.0398912058024 | | 1995 | 2.8558476881233 | | 1994 | 2.3912058023572 | | 1993 | 2.5951949229374 | | 1992 | 2.943109700816 | | 1991 | 1.693109700816 | | 1990 | 2.0308250226655 | | 1989 | 3.3182230281052 | | 1988 | 3.4904805077063 | | 1987 | 3.4338168631006 | | 1986 | 3.1368993653672 | | 1985 | 3.1368993653672 | | 1984 | 2.4036718041704 | | 1983 | 3.4451495920218 | | 1982 | 3.3556210335449 | | 1981 | 3.2932910244787 | | 1980 | 3.3884859474161 | | 1979 | 3.3658204895739 | | 1978 | 3.3544877606528 | | 1977 | 3.3318223028105 | | 1976 | 3.3204895738894 | | 1975 | 3.2978241160471 | | 1974 | 3.286491387126 | | 1973 | 3.2638259292838 | | 1972 | 3.2524932003626 | | 1971 | 3.2298277425204 | | 1970 | 3.2184950135993 | | 1969 | 3.195829555757 | | 1968 | 3.1844968268359 | | 1967 | 3.1618313689937 | | 1966 | 3.1504986400725 | | 1965 | 3.1278331822303 | | 1964 | 3.1165004533092 | | 1963 | 3.0938349954669 | | 1962 | 3.0825022665458 | | 1961 | 3.0598368087035 |
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