| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 7 |
1%
|
| 1888 | 1 |
0.1%
|
|
| 1901 | 1 |
0.1%
|
|
| 1922 | 2 |
0.3%
|
|
| 1929 | 1 |
0.1%
|
|
| 1931 | 1 |
0.1%
|
|
| 1938 | 1 |
0.1%
|
|
| 1940 | 1 |
0.1%
|
|
| 1944 | 1 |
0.1%
|
|
| 1948 | 1 |
0.1%
|
|
| 1950 | 2 |
0.3%
|
|
| 1952 | 2 |
0.3%
|
|
| 1953 | 2 |
0.3%
|
|
| 1955 | 3 |
0.4%
|
|
| 1956 | 1 |
0.1%
|
|
| 1958 | 2 |
0.3%
|
|
| 1959 | 2 |
0.3%
|
|
| 1960 | 1 |
0.1%
|
|
| 1962 | 4 |
0.5%
|
|
| 1963 | 2 |
0.3%
|
|
| 1964 | 4 |
0.5%
|
|
| 1965 | 5 |
0.7%
|
|
| 1966 | 4 |
0.5%
|
|
| 1969 | 4 |
0.5%
|
|
| 1970 | 4 |
0.5%
|
|
| 1972 | 6 |
0.8%
|
|
| 1973 | 5 |
0.7%
|
|
| 1974 | 6 |
0.8%
|
|
| 1975 | 3 |
0.4%
|
|
| 1976 | 3 |
0.4%
|
|
| 1977 | 5 |
0.7%
|
|
| 1978 | 5 |
0.7%
|
|
| 1979 | 4 |
0.5%
|
|
| 1980 | 10 |
1.4%
|
|
| 1981 | 8 |
1.1%
|
|
| 1982 | 8 |
1.1%
|
|
| 1983 | 12 |
1.6%
|
|
| 1984 | 7 |
1%
|
|
| 1985 | 10 |
1.4%
|
|
| 1986 | 9 |
1.2%
|
|
| 1987 | 9 |
1.2%
|
|
| 1988 | 17 |
2.3%
|
|
| 1989 | 11 |
1.5%
|
|
| 1990 | 17 |
2.3%
|
|
| 1991 | 15 |
2.1%
|
|
| 1992 | 27 |
3.7%
|
|
| 1993 | 11 |
1.5%
|
|
| 1994 | 17 |
2.3%
|
|
| 1995 | 18 |
2.5%
|
|
| 1996 | 14 |
1.9%
|
|
| 1997 | 20 |
2.7%
|
|
| 1998 | 17 |
2.3%
|
|
| 1999 | 13 |
1.8%
|
|
| 2000 | 35 |
4.8%
|
|
| 2001 | 25 |
3.4%
|
|
| 2002 | 15 |
2.1%
|
|
| 2003 | 24 |
3.3%
|
|
| 2004 | 12 |
1.6%
|
|
| 2005 | 17 |
2.3%
|
|
| 2006 | 12 |
1.6%
|
|
| 2007 | 20 |
2.7%
|
|
| 2008 | 26 |
3.6%
|
|
| 2009 | 20 |
2.7%
|
|
| 2010 | 16 |
2.2%
|
|
| 2011 | 15 |
2.1%
|
|
| 2012 | 16 |
2.2%
|
|
| 2013 | 21 |
2.9%
|
|
| 2014 | 16 |
2.2%
|
|
| 2015 | 18 |
2.5%
|
|
| 2016 | 12 |
1.6%
|
|
| 2017 | 14 |
1.9%
|
|
| 2018 | 10 |
1.4%
|
|
| 2019 | 8 |
1.1%
|
|
| 2020 | 3 |
0.4%
|
|
| 2021 | 6 |
0.8%
|
|
| 2022 | 2 |
0.3%
|