| Value | 
                    Category | 
                                                                            
                                    
                    
                        | 0 | 
                        no incentive  | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 1 | 
                        non-monetary incentive  | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 2 | 
                        monetary incentive  | 
                        
                            
                        
                        
                        
                        
                            
                    
                                    
                    
                        | 3 | 
                        monetary & non-monetary incentives  | 
                        
                            
                        
                        
                        
                        
                            
                    
                            
            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.