| Value | Category | 
                                    
                    
                        | -96 | Other (specify) | 
                                    
                    
                        | 1 | Every month | 
                                    
                    
                        | 2 | Once every several months | 
                                    
                    
                        | 3 | No fixed intervals of billing | 
                                    
                    
                        | 11 | .O | 
                                    
                    
                        | 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.