| Value | 
                    Category | 
                                    
                                                        
                        | 0 | 
                        Neither certificate or registered | 
                                            
                                                        
                        | 1 | 
                        Has certificate | 
                                            
                                                        
                        | 2 | 
                        Registered | 
                                            
                                                        
                        | 8 | 
                        Don't know | 
                                            
                            
            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.