| Value | Category | 
                                    
                    
                        | 1 | Not available | 
                                    
                    
                        | 2 | Available < 2 years | 
                                    
                    
                        | 3 | Available > 2 years | 
                                    
                    
                        | 7 | N/A | 
                                    
                    
                        | 8 | Invalid | 
                                    
                    
                        | 9 | Missing | 
                            
            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.