Annual % of total
1960 80.48977612543442
1961 79.74693021862834
1962 78.27821293878735
1963 78.0883502736513
1964 76.99359464065346
1965 74.86090338019743
1966 71.68399168399169
1967 66.40279249516924
1968 63.01493074293937
1969 59.81670778991893
1970 65.99790466212677
1971 60.985550203779184
1972 58.276117038132455
1973 54.77767740640369
1974 52.07343626286619
1975 49.28741670314704
1976 46.89275893675527
1977 44.23093873953284
1978 43.178510540978834
1979 43.7890708058686
1980 42.22569341833073
1981 43.550230726895556
1982 44.49134655972984
1983 41.537765344742475
1984 40.5795931546658
1985 42.434308589923766
1986 42.504718844651244
1987 42.40858160340484
1988 42.13559480860235
1989 40.63132530120482
1990 35.739924354790865
1991 36.709766072811775
1992 35.42448448275862
1993 32.781899623158615
1994 32.25284527518173
1995 31.715112310639043
1996 31.921592541909
1997 31.728333040884365
1998 29.18893635571055
1999 28.102479709894666
2000 27.722332051986086
2001 24.959047957371226
2002 25.47717957936365
2003 25.214954490812296
2004 23.68365667671573
2005 21.44107219442459
2006 21.971584926072847
2007 23.032790568139617
2008 22.89541203092589
2009 21.08678302675585
2010 22.65452380952381
2011 23.513826476231078
2012 24.49451300944048
2013 22.91425171736997
2014 21.66084174255476
2015 21.428872752420467
2016 19.9426101618705