Quality of included studies as assessed with the Newcastle–Ottawa Scale (NOS)
| Study | Selection of subjects | Comparability of cases and controls | Outcome measurements | Total score* | Quality† |
| Aite, 201457 | 3 | 1 | 2 | 6 | Good |
| Beers, 200025 | 4 | 2 | 3 | 9 | Good |
| Bevilacqua, 201446 | 3 | 1 | 3 | 7 | Good |
| Bevilacqua, 201558 | 3 | 1 | 2 | 6 | Good |
| Bouman, 199965 | 3 | 1 | 2 | 6 | Good |
| Burnett, 201866 | 3 | 1 | 2 | 6 | Good |
| Chesley, 201626 | 3 | 1 | 3 | 7 | Good |
| Costerus, 201967 | 3 | 1 | 2 | 6 | Good |
| Danzer, 201962 | 3 | 1 | 2 | 6 | Good |
| Doberschuetz, 201668 | 4 | 2 | 3 | 9 | Good |
| Elsinga, 201349 | 3 | 1 | 2 | 6 | Good |
| Faugli, 200969 | 2 | 1 | 2 | 5 | Fair |
| Gischler, 20098 | 3 | 1 | 3 | 7 | Good |
| Giudici, 201670 | 3 | 0 | 3 | 6 | Poor |
| Giudici, 201671 | 3 | 0 | 2 | 5 | Poor |
| Gorra, 201272 | 3 | 2 | 2 | 7 | Good |
| Harmsen, 201759 | 3 | 1 | 3 | 7 | Good |
| Harris, 201673 | 3 | 1 | 2 | 6 | Good |
| Hijkoop, 201774 | 3 | 1 | 3 | 7 | Good |
| Huang, 200822 | 3 | 1 | 3 | 7 | Good |
| Kato, 199360 | 2 | 1 | 3 | 6 | Fair |
| Konig, 201875 | 2 | 1 | 3 | 6 | Fair |
| Kubota, 201147 | 2 | 1 | 2 | 5 | Fair |
| Kumari, 201976 | 3 | 0 | 1 | 4 | Poor |
| Laing, 201177 | 1 | 1 | 3 | 5 | Poor |
| Ludman, 199061 | 3 | 2 | 3 | 8 | Good |
| Ludman, 199378 | 3 | 2 | 3 | 8 | Good |
| Maheshwari, 201379 | 3 | 1 | 3 | 7 | Good |
| Mazer, 20103 | 3 | 1 | 3 | 7 | Good |
| Mawlana, 201880 | 3 | 1 | 3 | 7 | Good |
| Minutillo, 201381 | 3 | 1 | 3 | 7 | Good |
| Moran, 201982 | 3 | 2 | 2 | 7 | Good |
| More, 201483 | 3 | 1 | 2 | 6 | Good |
| Newton, 201684 | 4 | 2 | 2 | 8 | Good |
| Payne, 201085 | 4 | 2 | 3 | 9 | Good |
| Plummer, 201986 | 2 | 1 | 2 | 5 | Fair |
| Sirichaipornsak, 201187 | 3 | 1 | 2 | 6 | Good |
| So, 201688 | 3 | 1 | 3 | 7 | Good |
| So, 201989 | 2 | 1 | 2 | 5 | Fair |
| So, 201990 | 2 | 1 | 3 | 6 | Fair |
| South, 200891 | 3 | 1 | 3 | 7 | Good |
| Van den Hondel, 201392 | 3 | 1 | 3 | 7 | Good |
| Van den Hondel, 201693 | 3 | 1 | 3 | 7 | Good |
| Van der Cammen-van Zijp, 20108 | 3 | 1 | 2 | 6 | Good |
| Van Eijck, 201350 | 3 | 1 | 2 | 6 | Good |
| Walker, 201394 | 3 | 2 | 3 | 8 | Good |
| Walker, 201595 | 3 | 2 | 2 | 7 | Good |
*The NOS allows study quality of observational studies to be quantified on the basis of the methods used to select subjects (4 points), comparability of case and control groups (2 points) and outcome measurements (3 points).
†Scores were converted to the Agency for Healthcare Research and Quality standards, in order to judge quality as ‘good’, ‘fair’ or ‘poor’.