A World Cancer Research Fund (WCRF) panel has come up with criteria for grading evidence to support a judgement of a relationship with cancer. The criteria are derived from human studies and biological evidence. The list of grades are shown in Table 1.
Table 1. WCRF criteria for grading evidence
|Convincing||Evidence strong enough to support a judgement of a convincing causal relationship.||Justifies goals and recommendations designed to reduce the incidence of cancer. A convincing relationship should be robust enough to be highly unlikely to be modified in the foreseeable future as new evidence accumulates.|
|Probable||Evidence strong enough to support a judgement of a probable causal relationship||Generally justify goals and recommendations designed to reduce the incidence of cancer.|
|Limited - suggestive||Evidence that is too limited to permit a probable or convincing causal judgement, but where there is evidence suggestive of a direction of effect. The evidence may have methodological flaws, or be limited in amount, but shows a generally consistent direction of effect.||Almost always does not justify recommendations designed to reduce the incidence of cancer. Any exceptions to this require special explicit justification.|
|Limited - no conclusion||Evidence is so limited that no firm conclusion can be made. This category represents an entry level, and is intended to allow any exposure for which there are sufficient data to warrant panel consideration, but where insufficient evidence exists to permit a more definitive grading. This does not necessarily mean a limited quantity of evidence.
A body of evidence for a particular exposure might be graded ‘limited — no conclusion’ for a number of reasons. The evidence might be limited by the amount of evidence in terms of the number of studies available, by inconsistency of direction of effect, by poor quality of studies (for example, lack of adjustment for known confounders), or by any combination of these factors.
|Does not necessarily indicate that there is evidence of no relationship. With further good quality research, any exposure graded in this way might in the future be shown to increase or decrease the risk of cancer.|
|Substantial effect on risk unlikely||Evidence is strong enough to support a judgement that a particular food, nutrition, or physical activity exposure is unlikely to have a substantial causal relation to a cancer outcome.||Evidence should be robust enough to be unlikely to be modified in the foreseeable future as new evidence accumulates.|
- World Cancer Research Fund, American Institute for Cancer Research. Food, nutrition, physical activity, and the prevention of cancer: a global perspective. Washington DC: AICR; 2007.