| Value | Category | 
                                    
                    
                        | -9 | don't know | 
                                    
                    
                        | -8 | refusal | 
                                    
                    
                        | -7 | does not apply | 
                                    
                    
                        | 1 | less than 1 year | 
                                    
                    
                        | 7 |  | 
                                    
                    
                        | 10 |  | 
                                    
                    
                        | 12 |  | 
                                    
                    
                        | 19 |  | 
                                    
                    
                        | 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.