| Value | Category | Cases | |
|---|---|---|---|
| 1 | 1. Per month | 7 | 
                                    
                                    70%
                                 | 
| 2 | 2. Per semester | 1 | 
                                    
                                    10%
                                 | 
| 3 | 3. Per year | 1 | 
                                    
                                    10%
                                 | 
| 5 | 5. Others | 1 | 
                                    
                                    10%
                                 | 
| 6 | 6. None | 0 | 
                                    
                                    0%
                                 | 
| 9 | 9. Missing | 0 | 
                                    
                                    0%
                                 | 
| Sysmiss | 329 |