| Value | Category | 
                                    
                    
                        | 1 | Month and year | 
                                    
                    
                        | 2 | Month and age -y imp | 
                                    
                    
                        | 5 | Year - a, m imp | 
                                    
                    
                        | 6 | Age - y, m imp | 
                                    
                    
                        | 7 | Month - a, y imp | 
                                    
                    
                        | 8 | None - all imp | 
                            
            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.