Newcastle–Ottawa scale summary table
| First author [ref.] | Selection | Comparability | Outcome |
| Sack [24] | **** | ** | *** |
| Sesé [14] | **** | ** | *** |
| Winterbottom [15] | **** | ** | *** |
| Singh [23] | **** | ** | *** |
| Conti [17] | **** | ** | *** |
| Johannson [13] | *** | ** | ** |
| Johannson [16] | **** | ** | *** |
A study can be awarded a maximum of four stars for selection, two for comparability and three for outcome.