| Value | Category | 
                                    
                    
                        | 1 | 1=Extremely useful | 
                                    
                    
                        | 2 | 2=Very useful | 
                                    
                    
                        | 3 | 3=Moderately useful | 
                                    
                    
                        | 4 | 4=Slightly useful | 
                                    
                    
                        | 5 | 5=Not at all useful | 
                                    
                    
                        | 6 | 6=Don't know | 
                                    
                    
                        | 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.