| Value | 
                    Category | 
                                                                            
                                    
                    
                        | -8 | 
                        Don't know  | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 1988 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 1991 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 1994 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 1996 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 1997 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2000 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2001 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2002 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2003 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2004 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2005 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2006 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2007 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2008 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2009 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2010 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2011 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2012 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2013 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2014 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2015 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2016 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2017 | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | Sysmiss | 
                          | 
                        
                            
                        
                        
                        
                        
                            
                    
                            
            Warning: these figures indicate the number of cases found in the data file. They cannot be interpreted as summary statistics of the population of interest.