| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 0 | 
                                    
                                    0%
                                 | 
| 1911 | 1 | 
                                    
                                    0.7%
                                 | |
| 1940 | 1 | 
                                    
                                    0.7%
                                 | |
| 1950 | 2 | 
                                    
                                    1.3%
                                 | |
| 1953 | 1 | 
                                    
                                    0.7%
                                 | |
| 1964 | 1 | 
                                    
                                    0.7%
                                 | |
| 1975 | 1 | 
                                    
                                    0.7%
                                 | |
| 1976 | 1 | 
                                    
                                    0.7%
                                 | |
| 1977 | 1 | 
                                    
                                    0.7%
                                 | |
| 1979 | 1 | 
                                    
                                    0.7%
                                 | |
| 1980 | 1 | 
                                    
                                    0.7%
                                 | |
| 1981 | 1 | 
                                    
                                    0.7%
                                 | |
| 1988 | 1 | 
                                    
                                    0.7%
                                 | |
| 1989 | 1 | 
                                    
                                    0.7%
                                 | |
| 1991 | 5 | 
                                    
                                    3.3%
                                 | |
| 1994 | 4 | 
                                    
                                    2.7%
                                 | |
| 1995 | 2 | 
                                    
                                    1.3%
                                 | |
| 1996 | 2 | 
                                    
                                    1.3%
                                 | |
| 1997 | 5 | 
                                    
                                    3.3%
                                 | |
| 1999 | 2 | 
                                    
                                    1.3%
                                 | |
| 2000 | 5 | 
                                    
                                    3.3%
                                 | |
| 2001 | 3 | 
                                    
                                    2%
                                 | |
| 2002 | 4 | 
                                    
                                    2.7%
                                 | |
| 2003 | 2 | 
                                    
                                    1.3%
                                 | |
| 2004 | 2 | 
                                    
                                    1.3%
                                 | |
| 2005 | 6 | 
                                    
                                    4%
                                 | |
| 2006 | 6 | 
                                    
                                    4%
                                 | |
| 2007 | 2 | 
                                    
                                    1.3%
                                 | |
| 2008 | 7 | 
                                    
                                    4.7%
                                 | |
| 2009 | 7 | 
                                    
                                    4.7%
                                 | |
| 2010 | 4 | 
                                    
                                    2.7%
                                 | |
| 2011 | 1 | 
                                    
                                    0.7%
                                 | |
| 2012 | 10 | 
                                    
                                    6.7%
                                 | |
| 2013 | 5 | 
                                    
                                    3.3%
                                 | |
| 2014 | 2 | 
                                    
                                    1.3%
                                 | |
| 2015 | 4 | 
                                    
                                    2.7%
                                 | |
| 2016 | 4 | 
                                    
                                    2.7%
                                 | |
| 2017 | 3 | 
                                    
                                    2%
                                 | |
| 2018 | 9 | 
                                    
                                    6%
                                 | |
| 2019 | 5 | 
                                    
                                    3.3%
                                 | |
| 2020 | 9 | 
                                    
                                    6%
                                 | |
| 2021 | 7 | 
                                    
                                    4.7%
                                 | |
| 2022 | 7 | 
                                    
                                    4.7%
                                 | |
| 2023 | 2 | 
                                    
                                    1.3%
                                 |