| Value | Category | 
                                    
                    
                        | 1 | Birth certificate | 
                                    
                    
                        | 2 | Immunization card | 
                                    
                    
                        | 3 | Parental information | 
                                    
                    
                        | 4 | Other relative/guardian | 
                                    
                    
                        | 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.