| 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.