| Value | Category | 
                                    
                    
                        | -9 | Don’t know | 
                                    
                    
                        | 1 | Professional and managers | 
                                    
                    
                        | 2 | Technicians | 
                                    
                    
                        | 3 | Both categories require the same amount of training | 
                            
            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.