| Value | Category | 
                                    
                    
                        | 0 | Never | 
                                    
                    
                        | 1 | 0-11 months ago | 
                                    
                    
                        | 2 | 12+ months ago | 
                                    
                    
                        | 3 | Yes, but don't remember when | 
                                    
                    
                        | 4 | Yes, but frequency in last 12 months missing | 
                            
            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.