| Value | Category | 
                                    
                    
                        | 1 | NO TT | 
                                    
                    
                        | 2 | LESS THAN 1 YR | 
                                    
                    
                        | 3 | 1 YEAR | 
                                    
                    
                        | 4 | 2 YEARS | 
                                    
                    
                        | 5 | 3 YEARS | 
                                    
                    
                        | 6 | MORE THAN 3 YRS | 
                                    
                    
                        | 9 | omitted | 
                            
            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.