| Value | Category | 
                                    
                    
                        | 1 | monthly or more often | 
                                    
                    
                        | 2 | every 2-3 months | 
                                    
                    
                        | 3 | every 4-6 months | 
                                    
                    
                        | 4 | less often than every 6 months | 
                                    
                    
                        | 8 | don't know | 
                            
            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.