| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 0 | 
                                    
                                    0%
                                 | 
| 1975 | 2 | 
                                    
                                    0.6%
                                 | |
| 1980 | 1 | 
                                    
                                    0.3%
                                 | |
| 1982 | 1 | 
                                    
                                    0.3%
                                 | |
| 1986 | 1 | 
                                    
                                    0.3%
                                 | |
| 1987 | 2 | 
                                    
                                    0.6%
                                 | |
| 1988 | 2 | 
                                    
                                    0.6%
                                 | |
| 1989 | 3 | 
                                    
                                    0.8%
                                 | |
| 1990 | 8 | 
                                    
                                    2.2%
                                 | |
| 1991 | 6 | 
                                    
                                    1.7%
                                 | |
| 1992 | 5 | 
                                    
                                    1.4%
                                 | |
| 1993 | 1 | 
                                    
                                    0.3%
                                 | |
| 1994 | 5 | 
                                    
                                    1.4%
                                 | |
| 1995 | 3 | 
                                    
                                    0.8%
                                 | |
| 1996 | 10 | 
                                    
                                    2.8%
                                 | |
| 1997 | 4 | 
                                    
                                    1.1%
                                 | |
| 1998 | 6 | 
                                    
                                    1.7%
                                 | |
| 1999 | 2 | 
                                    
                                    0.6%
                                 | |
| 2000 | 24 | 
                                    
                                    6.6%
                                 | |
| 2001 | 3 | 
                                    
                                    0.8%
                                 | |
| 2002 | 9 | 
                                    
                                    2.5%
                                 | |
| 2003 | 11 | 
                                    
                                    3%
                                 | |
| 2004 | 16 | 
                                    
                                    4.4%
                                 | |
| 2005 | 13 | 
                                    
                                    3.6%
                                 | |
| 2006 | 10 | 
                                    
                                    2.8%
                                 | |
| 2007 | 10 | 
                                    
                                    2.8%
                                 | |
| 2008 | 12 | 
                                    
                                    3.3%
                                 | |
| 2009 | 19 | 
                                    
                                    5.2%
                                 | |
| 2010 | 24 | 
                                    
                                    6.6%
                                 | |
| 2011 | 24 | 
                                    
                                    6.6%
                                 | |
| 2012 | 21 | 
                                    
                                    5.8%
                                 | |
| 2013 | 20 | 
                                    
                                    5.5%
                                 | |
| 2014 | 18 | 
                                    
                                    5%
                                 | |
| 2015 | 11 | 
                                    
                                    3%
                                 | |
| 2016 | 16 | 
                                    
                                    4.4%
                                 | |
| 2017 | 11 | 
                                    
                                    3%
                                 | |
| 2018 | 10 | 
                                    
                                    2.8%
                                 | |
| 2019 | 9 | 
                                    
                                    2.5%
                                 | |
| 2020 | 3 | 
                                    
                                    0.8%
                                 | |
| 2021 | 3 | 
                                    
                                    0.8%
                                 | |
| 2022 | 4 | 
                                    
                                    1.1%
                                 |