| Value | Category | 
                                    
                    
                        | 0 | NR | 
                                    
                    
                        | 1 | Electricity | 
                                    
                    
                        | 2 | Steam | 
                                    
                    
                        | 3 | Diesel | 
                                    
                    
                        | 4 | Others | 
                                    
                    
                        | 5 | No  motive power | 
                                    
                    
                        | 6 | Closed factory | 
                                    
                    
                        | 9 | Invalid | 
                            
            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.