| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 1 | 
                                    
                                    0.3%
                                 | 
| 1936 | 1 | 
                                    
                                    0.3%
                                 | |
| 1939 | 1 | 
                                    
                                    0.3%
                                 | |
| 1958 | 1 | 
                                    
                                    0.3%
                                 | |
| 1960 | 1 | 
                                    
                                    0.3%
                                 | |
| 1965 | 1 | 
                                    
                                    0.3%
                                 | |
| 1966 | 1 | 
                                    
                                    0.3%
                                 | |
| 1967 | 1 | 
                                    
                                    0.3%
                                 | |
| 1972 | 1 | 
                                    
                                    0.3%
                                 | |
| 1982 | 1 | 
                                    
                                    0.3%
                                 | |
| 1990 | 2 | 
                                    
                                    0.5%
                                 | |
| 1991 | 1 | 
                                    
                                    0.3%
                                 | |
| 1992 | 3 | 
                                    
                                    0.8%
                                 | |
| 1993 | 2 | 
                                    
                                    0.5%
                                 | |
| 1994 | 5 | 
                                    
                                    1.4%
                                 | |
| 1995 | 1 | 
                                    
                                    0.3%
                                 | |
| 1996 | 1 | 
                                    
                                    0.3%
                                 | |
| 1997 | 2 | 
                                    
                                    0.5%
                                 | |
| 1998 | 1 | 
                                    
                                    0.3%
                                 | |
| 2000 | 11 | 
                                    
                                    3%
                                 | |
| 2001 | 5 | 
                                    
                                    1.4%
                                 | |
| 2002 | 2 | 
                                    
                                    0.5%
                                 | |
| 2003 | 8 | 
                                    
                                    2.2%
                                 | |
| 2004 | 7 | 
                                    
                                    1.9%
                                 | |
| 2005 | 5 | 
                                    
                                    1.4%
                                 | |
| 2006 | 17 | 
                                    
                                    4.7%
                                 | |
| 2007 | 12 | 
                                    
                                    3.3%
                                 | |
| 2008 | 8 | 
                                    
                                    2.2%
                                 | |
| 2009 | 10 | 
                                    
                                    2.7%
                                 | |
| 2010 | 17 | 
                                    
                                    4.7%
                                 | |
| 2011 | 10 | 
                                    
                                    2.7%
                                 | |
| 2012 | 14 | 
                                    
                                    3.8%
                                 | |
| 2013 | 32 | 
                                    
                                    8.8%
                                 | |
| 2014 | 16 | 
                                    
                                    4.4%
                                 | |
| 2015 | 20 | 
                                    
                                    5.5%
                                 | |
| 2016 | 14 | 
                                    
                                    3.8%
                                 | |
| 2017 | 14 | 
                                    
                                    3.8%
                                 | |
| 2018 | 19 | 
                                    
                                    5.2%
                                 | |
| 2019 | 18 | 
                                    
                                    4.9%
                                 | |
| 2020 | 17 | 
                                    
                                    4.7%
                                 | |
| 2021 | 15 | 
                                    
                                    4.1%
                                 | |
| 2022 | 28 | 
                                    
                                    7.7%
                                 | |
| 2023 | 17 | 
                                    
                                    4.7%
                                 |