Annual % of total
1960 67.09961093237008
1961 64.09146806938652
1962 60.7852492141895
1963 58.88048569858425
1964 57.11365150971446
1965 55.74380812284592
1966 54.61808796881006
1967 52.67535959416668
1968 50.735630892240835
1969 49.4270462924361
1970 50.09528339426497
1971 48.18710898141856
1972 47.19807280960923
1973 45.76394507146424
1974 44.19378232212704
1975 43.32138441161039
1976 42.85707556612772
1977 42.13464337202867
1978 40.51660930966834
1979 40.26105327517592
1980 40.82177794537657
1981 37.77369508031387
1982 37.600755713824874
1983 36.8417508696544
1984 36.33705470352582
1985 38.39270329678698
1986 39.3740340030912
1987 39.639309042627715
1988 39.06025328864008
1989 37.973598393688675
1990 57.08820127767488
1991 51.496432647495595
1992 30.745727749161027
1993 30.22946629413928
1994 32.06007533155183
1995 31.82315815707074
1996 32.847131349470146
1997 31.782842277129202
1998 30.14803378192611
1999 31.53823981045909
2000 31.173069754951328
2001 29.967434603798647
2002 30.043087040463355
2003 29.549632700254243
2004 28.556620277422883
2005 28.326577914230654
2006 28.261283889591155
2007 26.93793761899077
2008 28.13842946055519
2009 24.652682036156424
2010 24.923583276485004
2011 24.10080792884738
2012 27.407572381603735
2013 25.963124099456902
2014 25.194964153691046
2015 29.09034516189094
2016 28.85286334835718