| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 3 | 
                                                                 
                                    
                                    0.8%
                                 
                                                             | 
                                                        
                    
| -7 | Never regiestered (spontaneous) | 0 | 
                                                                 
                                    
                                    0%
                                 
                                                             | 
                                                        
                    
| 1948 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1952 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1964 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1965 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1966 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1968 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1970 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1973 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1975 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1978 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1980 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1981 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1982 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1983 | 3 | 
                                                                 
                                    
                                    0.8%
                                 
                                                             | 
                                                        
                    |
| 1986 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1987 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1988 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1989 | 3 | 
                                                                 
                                    
                                    0.8%
                                 
                                                             | 
                                                        
                    |
| 1990 | 5 | 
                                                                 
                                    
                                    1.3%
                                 
                                                             | 
                                                        
                    |
| 1991 | 4 | 
                                                                 
                                    
                                    1.1%
                                 
                                                             | 
                                                        
                    |
| 1992 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1993 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1994 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1995 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1996 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 1997 | 1 | 
                                                                 
                                    
                                    0.3%
                                 
                                                             | 
                                                        
                    |
| 1998 | 4 | 
                                                                 
                                    
                                    1.1%
                                 
                                                             | 
                                                        
                    |
| 1999 | 2 | 
                                                                 
                                    
                                    0.5%
                                 
                                                             | 
                                                        
                    |
| 2000 | 13 | 
                                                                 
                                    
                                    3.4%
                                 
                                                             | 
                                                        
                    |
| 2001 | 7 | 
                                                                 
                                    
                                    1.8%
                                 
                                                             | 
                                                        
                    |
| 2002 | 3 | 
                                                                 
                                    
                                    0.8%
                                 
                                                             | 
                                                        
                    |
| 2003 | 4 | 
                                                                 
                                    
                                    1.1%
                                 
                                                             | 
                                                        
                    |
| 2004 | 7 | 
                                                                 
                                    
                                    1.8%
                                 
                                                             | 
                                                        
                    |
| 2005 | 12 | 
                                                                 
                                    
                                    3.2%
                                 
                                                             | 
                                                        
                    |
| 2006 | 18 | 
                                                                 
                                    
                                    4.7%
                                 
                                                             | 
                                                        
                    |
| 2007 | 9 | 
                                                                 
                                    
                                    2.4%
                                 
                                                             | 
                                                        
                    |
| 2008 | 16 | 
                                                                 
                                    
                                    4.2%
                                 
                                                             | 
                                                        
                    |
| 2009 | 11 | 
                                                                 
                                    
                                    2.9%
                                 
                                                             | 
                                                        
                    |
| 2010 | 20 | 
                                                                 
                                    
                                    5.3%
                                 
                                                             | 
                                                        
                    |
| 2011 | 12 | 
                                                                 
                                    
                                    3.2%
                                 
                                                             | 
                                                        
                    |
| 2012 | 22 | 
                                                                 
                                    
                                    5.8%
                                 
                                                             | 
                                                        
                    |
| 2013 | 11 | 
                                                                 
                                    
                                    2.9%
                                 
                                                             | 
                                                        
                    |
| 2014 | 13 | 
                                                                 
                                    
                                    3.4%
                                 
                                                             | 
                                                        
                    |
| 2015 | 16 | 
                                                                 
                                    
                                    4.2%
                                 
                                                             | 
                                                        
                    |
| 2016 | 13 | 
                                                                 
                                    
                                    3.4%
                                 
                                                             | 
                                                        
                    |
| 2017 | 11 | 
                                                                 
                                    
                                    2.9%
                                 
                                                             | 
                                                        
                    |
| 2018 | 26 | 
                                                                 
                                    
                                    6.8%
                                 
                                                             | 
                                                        
                    |
| 2019 | 25 | 
                                                                 
                                    
                                    6.6%
                                 
                                                             | 
                                                        
                    |
| 2020 | 41 | 
                                                                 
                                    
                                    10.8%
                                 
                                                             | 
                                                        
                    |
| 2021 | 7 | 
                                                                 
                                    
                                    1.8%
                                 
                                                             | 
                                                        
                    |
| 2022 | 7 | 
                                                                 
                                    
                                    1.8%
                                 
                                                             | 
                                                        
                    |
| 2023 | 3 | 
                                                                 
                                    
                                    0.8%
                                 
                                                             |