| Value | Category | 
                                    
                    
                        | 1 | sharp edges | 
                                    
                    
                        | 2 | old/broken | 
                                    
                    
                        | 3 | debris | 
                                    
                    
                        | 4 | not bolted to ground | 
                                    
                    
                        | 5 | hard landing | 
                                    
                    
                        | 6 | other | 
                                    
                    
                        | 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.