| Value | Category | Cases | |
|---|---|---|---|
| -9 | Don't know (spontaneous) | 13 | 
                                    
                                    8.1%
                                 | 
| 1958 | 1 | 
                                    
                                    0.6%
                                 | |
| 1960 | 1 | 
                                    
                                    0.6%
                                 | |
| 1970 | 1 | 
                                    
                                    0.6%
                                 | |
| 1975 | 1 | 
                                    
                                    0.6%
                                 | |
| 1980 | 1 | 
                                    
                                    0.6%
                                 | |
| 1983 | 1 | 
                                    
                                    0.6%
                                 | |
| 1985 | 1 | 
                                    
                                    0.6%
                                 | |
| 1990 | 4 | 
                                    
                                    2.5%
                                 | |
| 1991 | 3 | 
                                    
                                    1.9%
                                 | |
| 1992 | 5 | 
                                    
                                    3.1%
                                 | |
| 1993 | 1 | 
                                    
                                    0.6%
                                 | |
| 1994 | 3 | 
                                    
                                    1.9%
                                 | |
| 1995 | 6 | 
                                    
                                    3.7%
                                 | |
| 1996 | 3 | 
                                    
                                    1.9%
                                 | |
| 1997 | 2 | 
                                    
                                    1.2%
                                 | |
| 1998 | 7 | 
                                    
                                    4.3%
                                 | |
| 1999 | 2 | 
                                    
                                    1.2%
                                 | |
| 2000 | 5 | 
                                    
                                    3.1%
                                 | |
| 2001 | 1 | 
                                    
                                    0.6%
                                 | |
| 2002 | 5 | 
                                    
                                    3.1%
                                 | |
| 2003 | 9 | 
                                    
                                    5.6%
                                 | |
| 2004 | 8 | 
                                    
                                    5%
                                 | |
| 2005 | 7 | 
                                    
                                    4.3%
                                 | |
| 2006 | 5 | 
                                    
                                    3.1%
                                 | |
| 2007 | 7 | 
                                    
                                    4.3%
                                 | |
| 2008 | 7 | 
                                    
                                    4.3%
                                 | |
| 2009 | 5 | 
                                    
                                    3.1%
                                 | |
| 2010 | 8 | 
                                    
                                    5%
                                 | |
| 2011 | 7 | 
                                    
                                    4.3%
                                 | |
| 2012 | 3 | 
                                    
                                    1.9%
                                 | |
| 2013 | 5 | 
                                    
                                    3.1%
                                 | |
| 2014 | 7 | 
                                    
                                    4.3%
                                 | |
| 2015 | 8 | 
                                    
                                    5%
                                 | |
| 2016 | 2 | 
                                    
                                    1.2%
                                 | |
| 2017 | 3 | 
                                    
                                    1.9%
                                 | |
| 2018 | 3 | 
                                    
                                    1.9%
                                 | |
| Sysmiss | 200 |