| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 3 | 
                                    
                                    2.7%
                                 | 
| 1968 | 1 | 
                                    
                                    0.9%
                                 | |
| 1970 | 1 | 
                                    
                                    0.9%
                                 | |
| 1972 | 1 | 
                                    
                                    0.9%
                                 | |
| 1975 | 2 | 
                                    
                                    1.8%
                                 | |
| 1979 | 2 | 
                                    
                                    1.8%
                                 | |
| 1980 | 1 | 
                                    
                                    0.9%
                                 | |
| 1983 | 2 | 
                                    
                                    1.8%
                                 | |
| 1985 | 3 | 
                                    
                                    2.7%
                                 | |
| 1986 | 1 | 
                                    
                                    0.9%
                                 | |
| 1987 | 3 | 
                                    
                                    2.7%
                                 | |
| 1988 | 1 | 
                                    
                                    0.9%
                                 | |
| 1989 | 1 | 
                                    
                                    0.9%
                                 | |
| 1992 | 3 | 
                                    
                                    2.7%
                                 | |
| 1993 | 2 | 
                                    
                                    1.8%
                                 | |
| 1994 | 2 | 
                                    
                                    1.8%
                                 | |
| 1996 | 1 | 
                                    
                                    0.9%
                                 | |
| 1997 | 1 | 
                                    
                                    0.9%
                                 | |
| 1999 | 1 | 
                                    
                                    0.9%
                                 | |
| 2000 | 6 | 
                                    
                                    5.4%
                                 | |
| 2001 | 1 | 
                                    
                                    0.9%
                                 | |
| 2002 | 2 | 
                                    
                                    1.8%
                                 | |
| 2004 | 4 | 
                                    
                                    3.6%
                                 | |
| 2005 | 4 | 
                                    
                                    3.6%
                                 | |
| 2006 | 2 | 
                                    
                                    1.8%
                                 | |
| 2007 | 2 | 
                                    
                                    1.8%
                                 | |
| 2008 | 2 | 
                                    
                                    1.8%
                                 | |
| 2009 | 7 | 
                                    
                                    6.3%
                                 | |
| 2010 | 2 | 
                                    
                                    1.8%
                                 | |
| 2011 | 2 | 
                                    
                                    1.8%
                                 | |
| 2012 | 4 | 
                                    
                                    3.6%
                                 | |
| 2013 | 3 | 
                                    
                                    2.7%
                                 | |
| 2014 | 1 | 
                                    
                                    0.9%
                                 | |
| 2015 | 3 | 
                                    
                                    2.7%
                                 | |
| 2016 | 2 | 
                                    
                                    1.8%
                                 | |
| 2017 | 4 | 
                                    
                                    3.6%
                                 | |
| 2018 | 8 | 
                                    
                                    7.2%
                                 | |
| 2019 | 7 | 
                                    
                                    6.3%
                                 | |
| 2020 | 4 | 
                                    
                                    3.6%
                                 | |
| 2021 | 5 | 
                                    
                                    4.5%
                                 | |
| 2022 | 2 | 
                                    
                                    1.8%
                                 | |
| 2023 | 2 | 
                                    
                                    1.8%
                                 |