Annual % of total
1960 76.74469687460775
1961 75.26104417670683
1962 73.76074556117939
1963 72.81312247971627
1964 70.12176009644364
1965 67.87678897461569
1966 65.25989424011762
1967 62.239991088061096
1968 61.05774458984305
1969 58.55817506458065
1970 54.082159782584284
1971 51.70376042481647
1972 45.241568840333855
1973 44.86007789191802
1974 42.677662789524
1975 45.79108829693528
1976 45.94703811418949
1977 44.86223289432196
1978 42.861558227479875
1979 43.695696674703186
1980 45.041589122260625
1981 46.6492738840176
1982 44.65547555537728
1983 45.33781286746178
1984 33.56887032303205
1985 41.88466929536136
1986 43.72143101214001
1987 45.3767233087528
1988 43.8190618833043
1989 42.30223718300271
1990 42.415970109037346
1991 41.49855314373882
1992 40.670689175015376
1993 36.50039480500259
1994 34.703591027101744
1995 33.53029430277137
1996 30.29890225094554
1997 28.554341316792065
1998 27.77681084890387
1999 24.931050999310823
2000 25.91497924998114
2001 26.888953680575963
2002 25.409585840107123
2003 26.72862012987013
2004 25.93784481486945
2005 26.520970403255646
2006 28.768523558082858
2007 27.76999225187089
2008 26.419137847242474
2009 24.056409313725492
2010 24.135114884078128
2011 25.82461770397372
2012 31.181433373297935
2013 31.167800291352144
2014 27.602605520067495
2015 22.587994604990385
2016 11.880077331151522