| Value | Category | 
                                    
                    
                        | 1 | always (since birth) | 
                                    
                    
                        | 2 | less than one year | 
                                    
                    
                        | 3 | from one to three years | 
                                    
                    
                        | 4 | from three to five years | 
                                    
                    
                        | 5 | more than five years | 
                                    
                    
                        | 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.