| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 5 | 
                                    
                                    0.5%
                                 | 
| 1825 | 2 | 
                                    
                                    0.2%
                                 | |
| 1827 | 1 | 
                                    
                                    0.1%
                                 | |
| 1860 | 2 | 
                                    
                                    0.2%
                                 | |
| 1870 | 1 | 
                                    
                                    0.1%
                                 | |
| 1873 | 1 | 
                                    
                                    0.1%
                                 | |
| 1881 | 2 | 
                                    
                                    0.2%
                                 | |
| 1886 | 1 | 
                                    
                                    0.1%
                                 | |
| 1894 | 1 | 
                                    
                                    0.1%
                                 | |
| 1896 | 2 | 
                                    
                                    0.2%
                                 | |
| 1900 | 1 | 
                                    
                                    0.1%
                                 | |
| 1919 | 1 | 
                                    
                                    0.1%
                                 | |
| 1921 | 2 | 
                                    
                                    0.2%
                                 | |
| 1922 | 1 | 
                                    
                                    0.1%
                                 | |
| 1927 | 2 | 
                                    
                                    0.2%
                                 | |
| 1929 | 1 | 
                                    
                                    0.1%
                                 | |
| 1933 | 2 | 
                                    
                                    0.2%
                                 | |
| 1934 | 1 | 
                                    
                                    0.1%
                                 | |
| 1945 | 2 | 
                                    
                                    0.2%
                                 | |
| 1946 | 7 | 
                                    
                                    0.7%
                                 | |
| 1947 | 8 | 
                                    
                                    0.8%
                                 | |
| 1948 | 5 | 
                                    
                                    0.5%
                                 | |
| 1949 | 4 | 
                                    
                                    0.4%
                                 | |
| 1950 | 4 | 
                                    
                                    0.4%
                                 | |
| 1951 | 1 | 
                                    
                                    0.1%
                                 | |
| 1952 | 4 | 
                                    
                                    0.4%
                                 | |
| 1953 | 1 | 
                                    
                                    0.1%
                                 | |
| 1954 | 4 | 
                                    
                                    0.4%
                                 | |
| 1955 | 7 | 
                                    
                                    0.7%
                                 | |
| 1956 | 3 | 
                                    
                                    0.3%
                                 | |
| 1957 | 3 | 
                                    
                                    0.3%
                                 | |
| 1958 | 3 | 
                                    
                                    0.3%
                                 | |
| 1960 | 4 | 
                                    
                                    0.4%
                                 | |
| 1961 | 3 | 
                                    
                                    0.3%
                                 | |
| 1962 | 4 | 
                                    
                                    0.4%
                                 | |
| 1963 | 3 | 
                                    
                                    0.3%
                                 | |
| 1964 | 1 | 
                                    
                                    0.1%
                                 | |
| 1965 | 2 | 
                                    
                                    0.2%
                                 | |
| 1967 | 4 | 
                                    
                                    0.4%
                                 | |
| 1968 | 3 | 
                                    
                                    0.3%
                                 | |
| 1969 | 5 | 
                                    
                                    0.5%
                                 | |
| 1970 | 1 | 
                                    
                                    0.1%
                                 | |
| 1971 | 4 | 
                                    
                                    0.4%
                                 | |
| 1972 | 2 | 
                                    
                                    0.2%
                                 | |
| 1973 | 3 | 
                                    
                                    0.3%
                                 | |
| 1974 | 5 | 
                                    
                                    0.5%
                                 | |
| 1975 | 3 | 
                                    
                                    0.3%
                                 | |
| 1976 | 5 | 
                                    
                                    0.5%
                                 | |
| 1977 | 3 | 
                                    
                                    0.3%
                                 | |
| 1978 | 5 | 
                                    
                                    0.5%
                                 | |
| 1979 | 3 | 
                                    
                                    0.3%
                                 | |
| 1980 | 2 | 
                                    
                                    0.2%
                                 | |
| 1981 | 2 | 
                                    
                                    0.2%
                                 | |
| 1982 | 2 | 
                                    
                                    0.2%
                                 | |
| 1983 | 3 | 
                                    
                                    0.3%
                                 | |
| 1984 | 3 | 
                                    
                                    0.3%
                                 | |
| 1985 | 7 | 
                                    
                                    0.7%
                                 | |
| 1986 | 6 | 
                                    
                                    0.6%
                                 | |
| 1987 | 6 | 
                                    
                                    0.6%
                                 | |
| 1988 | 4 | 
                                    
                                    0.4%
                                 | |
| 1989 | 46 | 
                                    
                                    4.8%
                                 | |
| 1990 | 84 | 
                                    
                                    8.8%
                                 | |
| 1991 | 71 | 
                                    
                                    7.4%
                                 | |
| 1992 | 77 | 
                                    
                                    8.1%
                                 | |
| 1993 | 68 | 
                                    
                                    7.1%
                                 | |
| 1994 | 31 | 
                                    
                                    3.2%
                                 | |
| 1995 | 29 | 
                                    
                                    3%
                                 | |
| 1996 | 35 | 
                                    
                                    3.7%
                                 | |
| 1997 | 32 | 
                                    
                                    3.4%
                                 | |
| 1998 | 23 | 
                                    
                                    2.4%
                                 | |
| 1999 | 18 | 
                                    
                                    1.9%
                                 | |
| 2000 | 30 | 
                                    
                                    3.1%
                                 | |
| 2001 | 17 | 
                                    
                                    1.8%
                                 | |
| 2002 | 13 | 
                                    
                                    1.4%
                                 | |
| 2003 | 32 | 
                                    
                                    3.4%
                                 | |
| 2004 | 20 | 
                                    
                                    2.1%
                                 | |
| 2005 | 22 | 
                                    
                                    2.3%
                                 | |
| 2006 | 11 | 
                                    
                                    1.2%
                                 | |
| 2007 | 18 | 
                                    
                                    1.9%
                                 | |
| 2008 | 14 | 
                                    
                                    1.5%
                                 | |
| 2009 | 10 | 
                                    
                                    1%
                                 | |
| 2010 | 16 | 
                                    
                                    1.7%
                                 | |
| 2011 | 14 | 
                                    
                                    1.5%
                                 | |
| 2012 | 13 | 
                                    
                                    1.4%
                                 | |
| 2013 | 6 | 
                                    
                                    0.6%
                                 | |
| 2014 | 12 | 
                                    
                                    1.3%
                                 | |
| 2015 | 2 | 
                                    
                                    0.2%
                                 | |
| 2016 | 6 | 
                                    
                                    0.6%
                                 | |
| 2017 | 2 | 
                                    
                                    0.2%
                                 |