Annual % of total
1960 58.60797144556812
1961 55.4724364138566
1962 53.38524767467013
1963 53.023394131641545
1964 48.66290018832392
1965 47.08085269829076
1966 42.93853913693256
1967 39.44398568676025
1968 36.358126244696514
1969 34.3206432556613
1970 35.141782407407405
1971 28.87576460662308
1972 27.84521012790394
1973 26.372889620284028
1974 28.91781521184278
1975 26.798355019213915
1976 24.444444444444446
1977 22.32731067771182
1978 22.34626180147997
1979 23.456569625528367
1980 27.152271452607962
1981 26.10570327301235
1982 26.249149081007484
1983 27.715857787810382
1984 30.922290938424307
1985 30.475296641415834
1986 27.25115269867101
1987 25.64933003111704
1988 26.804619826756497
1989 25.606450738582463
1990 26.96601166981294
1991 26.33636815123358
1992 22.10882434541356
1993 19.794186851211073
1994 19.9234671470235
1995 21.601614906832296
1996 20.453836671802776
1997 21.966597633136097
1998 19.11466986237823
1999 19.51274385408406
2000 21.88194081536282
2001 21.107794139302015
2002 20.99128874153092
2003 21.004237174321783
2004 20.115284960422166
2005 20.0617221930745
2006 20.863794871794873
2007 20.620440763615946
2008 20.26104340449597
2009 17.037148401117665
2010 18.639010863350485
2011 19.41194961183536
2012 18.906648748272158
2013 19.057245627376425
2014 18.41485884820132
2015 19.42377672209026
2016 17.973448493453226