| Year | Percent |
| 1950 | 0.001657394495 |
| 1951 | 0.001347184035 |
| 1952 | 0.001712600307 |
| 1953 | 0.00108176217 |
| 1954 | 0.001298828543 |
| 1955 | 0.001456833212 |
| 1956 | 0.001935580371 |
| 1957 | 0.001933224579 |
| 1958 | 0.00147088904 |
| 1959 | 0.001485525653 |
| 1960 | 0.001488178105 |
| 1961 | 0.001496767944 |
| 1962 | 0.001540675792 |
| 1963 | 0.001638057226 |
| 1964 | 0.00154100773 |
| 1965 | 0.001544063763 |
| 1966 | 0.001662005806 |
| 1967 | 0.001843439053 |
| 1968 | 0.002291134205 |
| 1969 | 0.001788653102 |
| 1970 | 0.001839739539 |
| 1971 | 0.001802023059 |
| 1972 | 0.00184941941 |
| 1973 | 0.001896180863 |
| 1974 | 0.001856547974 |
| 1975 | 0.001798989106 |
| 1976 | 0.002307629775 |
| 1977 | 0.0021000636 |
| 1978 | 0.002669629081 |
| 1979 | 0.001763526456 |
| 1980 | 0.002317108154 |
| 1981 | 0.002599484303 |
| 1982 | 0.002307280373 |
| 1983 | 0.002353591421 |
| 1984 | 0.002294572596 |
| 1985 | 0.002381776132 |
| 1986 | 0.002490220183 |
| 1987 | 0.002465429596 |
| 1988 | 0.002609617677 |
| 1989 | 0.004388283316 |
| 1990 | 0.006106594108 |
| 1991 | 0.007162404244 |
| 1992 | 0.003763649849 |
| 1993 | 0.003445750973 |
| 1994 | 0.003024216565 |
| 1995 | 0.002674241277 |
| 1996 | 0.003358175417 |
| 1997 | 0.002889978522 |
| 1998 | 0.00233665392 |
| 1999 | 0.002278853292 |
| 2000 | 0.002938658792 |
| 2001 | 0.003660297011 |
| 2002 | 0.003625459192 |
| 2003 | 0.002385234213 |
| 2004 | 0.00321705071 |
| 2005 | 0.003732688164 |
| 2006 | 0.004171240851 |
| 2007 | 0.00343360367 |
| 2008 | 0.003577731881 |
| 2009 | 0.00362128211 |
| 2010 | 0.003310551058 |
| 2011 | 0.003344192039 |
| 2012 | 0.003220631155 |
| 2013 | 0.003066578023 |
| 2014 | 0.003121728261 |
| 2015 | 0.003202872844 |
| 2016 | 0.003579269762 |
| 2017 | 0.003382793816 |
| 2018 | 0.003268280199 |
| 2019 | 0.003055427178 |
