| Value | Category | 
                                    
                    
                        | 1 | Calendar | 
                                    
                    
                        | 2 | Body temperature | 
                                    
                    
                        | 3 | Cervical mucus | 
                                    
                    
                        | 4 | Body temp. & mucus | 
                                    
                    
                        | 5 | No specific system | 
                                    
                    
                        | 6 | Other | 
                            
            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.