Annual % of total
1960 15.541905762547485
1961 16.734052111410605
1962 17.4934725848564
1963 17.109721136304405
1964 17.60004701181172
1965 18.713730182470837
1966 20.671220671220674
1967 23.873340397681233
1968 25.616118006835762
1969 27.49970626248384
1970 22.718700890518598
1971 26.54951569364315
1972 28.308862364001264
1973 30.121449362641773
1974 32.06650831353919
1975 33.27496473563889
1976 33.17140238313473
1977 35.60599382988101
1978 36.209405796498814
1979 33.795449712949186
1980 34.01021657449233
1981 32.18746031074045
1982 31.00042211903757
1983 35.21474411716307
1984 36.05505327736519
1985 34.300072398960864
1986 32.096922874325095
1987 32.44593068704942
1988 31.15775664152181
1989 31.460240963855423
1990 34.380164639572826
1991 33.12513865220759
1992 36.107305172413795
1993 38.30940904419322
1994 33.60464693665628
1995 34.313874281734286
1996 31.789866575436193
1997 33.63936831022987
1998 32.31563731473409
1999 31.692859609739248
2000 30.95101043382757
2001 29.570479573712255
2002 27.599728563724607
2003 26.310709256397047
2004 27.396827451675833
2005 29.671508220157257
2006 30.825016227912013
2007 32.06752692164872
2008 31.44920988119932
2009 33.56562081939799
2010 33.07039682539683
2011 32.5972926881262
2012 33.275724153810735
2013 35.113594210009815
2014 38.7548560177209
2015 39.40883817427386
2016 38.85470098920864