| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 25 |
1.5%
|
| 1920 | 1 |
0.1%
|
|
| 1923 | 1 |
0.1%
|
|
| 1926 | 2 |
0.1%
|
|
| 1930 | 2 |
0.1%
|
|
| 1932 | 1 |
0.1%
|
|
| 1934 | 1 |
0.1%
|
|
| 1935 | 1 |
0.1%
|
|
| 1936 | 1 |
0.1%
|
|
| 1940 | 1 |
0.1%
|
|
| 1941 | 1 |
0.1%
|
|
| 1945 | 1 |
0.1%
|
|
| 1946 | 3 |
0.2%
|
|
| 1950 | 4 |
0.2%
|
|
| 1952 | 1 |
0.1%
|
|
| 1953 | 2 |
0.1%
|
|
| 1954 | 4 |
0.2%
|
|
| 1955 | 1 |
0.1%
|
|
| 1956 | 1 |
0.1%
|
|
| 1957 | 1 |
0.1%
|
|
| 1959 | 1 |
0.1%
|
|
| 1960 | 3 |
0.2%
|
|
| 1961 | 1 |
0.1%
|
|
| 1962 | 1 |
0.1%
|
|
| 1963 | 2 |
0.1%
|
|
| 1964 | 3 |
0.2%
|
|
| 1965 | 4 |
0.2%
|
|
| 1966 | 2 |
0.1%
|
|
| 1967 | 4 |
0.2%
|
|
| 1968 | 6 |
0.4%
|
|
| 1969 | 5 |
0.3%
|
|
| 1970 | 11 |
0.7%
|
|
| 1971 | 2 |
0.1%
|
|
| 1972 | 10 |
0.6%
|
|
| 1973 | 6 |
0.4%
|
|
| 1974 | 5 |
0.3%
|
|
| 1975 | 1 |
0.1%
|
|
| 1976 | 12 |
0.7%
|
|
| 1977 | 11 |
0.7%
|
|
| 1978 | 25 |
1.5%
|
|
| 1979 | 13 |
0.8%
|
|
| 1980 | 21 |
1.3%
|
|
| 1981 | 4 |
0.2%
|
|
| 1982 | 16 |
1%
|
|
| 1983 | 16 |
1%
|
|
| 1984 | 13 |
0.8%
|
|
| 1985 | 13 |
0.8%
|
|
| 1986 | 12 |
0.7%
|
|
| 1987 | 13 |
0.8%
|
|
| 1988 | 13 |
0.8%
|
|
| 1989 | 18 |
1.1%
|
|
| 1990 | 37 |
2.2%
|
|
| 1991 | 17 |
1%
|
|
| 1992 | 30 |
1.8%
|
|
| 1993 | 45 |
2.7%
|
|
| 1994 | 38 |
2.3%
|
|
| 1995 | 55 |
3.3%
|
|
| 1996 | 44 |
2.6%
|
|
| 1997 | 37 |
2.2%
|
|
| 1998 | 46 |
2.8%
|
|
| 1999 | 40 |
2.4%
|
|
| 2000 | 78 |
4.7%
|
|
| 2001 | 31 |
1.9%
|
|
| 2002 | 40 |
2.4%
|
|
| 2003 | 44 |
2.6%
|
|
| 2004 | 41 |
2.5%
|
|
| 2005 | 61 |
3.7%
|
|
| 2006 | 53 |
3.2%
|
|
| 2007 | 54 |
3.2%
|
|
| 2008 | 71 |
4.3%
|
|
| 2009 | 50 |
3%
|
|
| 2010 | 73 |
4.4%
|
|
| 2011 | 67 |
4%
|
|
| 2012 | 52 |
3.1%
|
|
| 2013 | 62 |
3.7%
|
|
| 2014 | 54 |
3.2%
|
|
| 2015 | 48 |
2.9%
|
|
| 2016 | 45 |
2.7%
|
|
| 2017 | 28 |
1.7%
|