| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 2 | 
                                                                 
                                    
                                    1%
                                 
                                                             | 
                                                        
                    
| 1939 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1954 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1958 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1960 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1967 | 2 | 
                                                                 
                                    
                                    1%
                                 
                                                             | 
                                                        
                    |
| 1968 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1975 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1977 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1980 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1981 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1983 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1985 | 2 | 
                                                                 
                                    
                                    1%
                                 
                                                             | 
                                                        
                    |
| 1986 | 2 | 
                                                                 
                                    
                                    1%
                                 
                                                             | 
                                                        
                    |
| 1990 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1991 | 2 | 
                                                                 
                                    
                                    1%
                                 
                                                             | 
                                                        
                    |
| 1992 | 4 | 
                                                                 
                                    
                                    1.9%
                                 
                                                             | 
                                                        
                    |
| 1993 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1994 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1995 | 5 | 
                                                                 
                                    
                                    2.4%
                                 
                                                             | 
                                                        
                    |
| 1996 | 3 | 
                                                                 
                                    
                                    1.4%
                                 
                                                             | 
                                                        
                    |
| 1997 | 2 | 
                                                                 
                                    
                                    1%
                                 
                                                             | 
                                                        
                    |
| 1998 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1999 | 4 | 
                                                                 
                                    
                                    1.9%
                                 
                                                             | 
                                                        
                    |
| 2000 | 13 | 
                                                                 
                                    
                                    6.2%
                                 
                                                             | 
                                                        
                    |
| 2001 | 4 | 
                                                                 
                                    
                                    1.9%
                                 
                                                             | 
                                                        
                    |
| 2002 | 7 | 
                                                                 
                                    
                                    3.3%
                                 
                                                             | 
                                                        
                    |
| 2003 | 8 | 
                                                                 
                                    
                                    3.8%
                                 
                                                             | 
                                                        
                    |
| 2004 | 6 | 
                                                                 
                                    
                                    2.9%
                                 
                                                             | 
                                                        
                    |
| 2005 | 10 | 
                                                                 
                                    
                                    4.8%
                                 
                                                             | 
                                                        
                    |
| 2006 | 1 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 2007 | 5 | 
                                                                 
                                    
                                    2.4%
                                 
                                                             | 
                                                        
                    |
| 2008 | 2 | 
                                                                 
                                    
                                    1%
                                 
                                                             | 
                                                        
                    |
| 2009 | 7 | 
                                                                 
                                    
                                    3.3%
                                 
                                                             | 
                                                        
                    |
| 2010 | 16 | 
                                                                 
                                    
                                    7.7%
                                 
                                                             | 
                                                        
                    |
| 2011 | 3 | 
                                                                 
                                    
                                    1.4%
                                 
                                                             | 
                                                        
                    |
| 2012 | 17 | 
                                                                 
                                    
                                    8.1%
                                 
                                                             | 
                                                        
                    |
| 2013 | 8 | 
                                                                 
                                    
                                    3.8%
                                 
                                                             | 
                                                        
                    |
| 2014 | 10 | 
                                                                 
                                    
                                    4.8%
                                 
                                                             | 
                                                        
                    |
| 2015 | 19 | 
                                                                 
                                    
                                    9.1%
                                 
                                                             | 
                                                        
                    |
| 2016 | 5 | 
                                                                 
                                    
                                    2.4%
                                 
                                                             | 
                                                        
                    |
| 2017 | 8 | 
                                                                 
                                    
                                    3.8%
                                 
                                                             | 
                                                        
                    |
| 2018 | 8 | 
                                                                 
                                    
                                    3.8%
                                 
                                                             | 
                                                        
                    |
| 2019 | 5 | 
                                                                 
                                    
                                    2.4%
                                 
                                                             | 
                                                        
                    |
| 2020 | 5 | 
                                                                 
                                    
                                    2.4%
                                 
                                                             |