Annual % of total
1960 33.46017939657516
1961 30.706865974157594
1962 28.546371409778555
1963 26.5370800760617
1964 25.15170537769408
1965 24.561403508771924
1966 23.082926829268292
1967 23.66717440974867
1968 22.097442985487213
1969 20.988457161437164
1970 19.32798096937259
1971 19.02828528894302
1972 15.465660770031217
1973 16.907571940061
1974 17.307148912122067
1975 15.519439133205863
1976 15.13768446675795
1977 16.677245318946365
1978 17.082134867369767
1979 17.09439528023599
1980 20.27576197387518
1981 20.101553166069298
1982 15.99075062255425
1983 19.408762977300718
1984 23.554674595308885
1985 23.53342428376535
1986 23.833855282676776
1987 23.095584667637066
1988 26.71932452676018
1989 30.548007246376812
1990 35.23503452811575
1991 29.214428281461437
1992 23.69750714512544
1993 23.636823529411764
1994 26.481585641605427
1995 26.977615686274508
1996 32.741665910122556
1997 36.82956091772152
1998 34.59433962264151
1999 34.46720848056537
2000 28.610254917703738
2001 19.365314393148108
2002 19.280180735263915
2003 18.53704868763775
2004 18.800661428832452
2005 18.413973438621678
2006 21.995572304995616
2007 20.10789311090516
2008 24.53800586080586
2009 20.564818724185404
2010 23.720840504876648
2011 27.285510657774292
2012 29.93307300293031
2013 30.64381118881119
2014 31.941638957978796
2015 33.16270708795901
2016 31.358467966573816