Grading criteria for evidence categories from meta-analyses of summary level data included in review. Evidence relates to the strength of associations between environmental risk factors and risk of non-Hodgkin's lymphoma
| Strength of association | Description* |
| Convincing | Highly significant association (P<0.000001) |
| At least 1000 cases of non-Hodgkin's lymphoma | |
| Low or moderate proportion of total variability due to variability between studies (I2 <50%) | |
| 95% prediction interval excluding the null value | |
| Largest study reporting a nominally significant result (P<0.05) | |
| No evidence of small study effects | |
| No evidence of excess significance bias | |
| Highly suggestive | Highly significant association (P<0.000001) |
| At least 1000 non-Hodgkin's lymphoma cases | |
| Largest study reporting a nominally significant result (P<0.05) | |
| Suggestive | At least 1000 non-Hodgkin's lymphoma cases |
| Significant association (P<0.001) | |
| Weak | Nominally significant association (P<0.05) |
| Non-significant | Non-significant associations (P>0.05) |
*P value for the association from a random effects model.