| Value | Category | 
                                    
                    
                        | 1 | Less than 12 months ago | 
                                    
                    
                        | 2 | 12-23 months ago | 
                                    
                    
                        | 3 | 2 or more years ago | 
                                    
                    
                        | 9 | Missing | 
                                    
                    
                        | 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.