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Table 4 Criteria for grading evidence, see World Cancer Research Fund [34] for the full list

From: Determinants of dietary behavior among youth: an umbrella review

Strength of evidence:
Ideally the definition of the strength of evidence should be based on a relationship that has been established by multiple randomized controlled trials of manipulations of single isolated variables, but this type of evidence is often not available.
The following criteria were used to describe the strength of evidence in this report. They are based on the criteria used by the World Cancer Research Fund (World Cancer Research Fund, 2007 [34]), but have been modified for the research question at hand. Four categories were defined: convincing/probable/limited, suggestive/limited, no conclusion.
Convincing evidence: Evidence based on studies of determinants showing consistent associations between the variable and the behavioral outcome. The available evidence is based on a substantial number of studies including longitudinal observational studies and where relevant, experimental studies of sufficient size, duration and quality showing consistent effects. Specifically, the grading criteria include evidence from more than one study type and evidence from at least two independent cohort studies should be available, and strong and plausible experimental evidence.
Probable evidence: Evidence based on studies of determinants showing fairly consistent associations between the variable and the behavioral outcome, but there are shortcomings in the available evidence or some evidence to the contrary, which precludes a more definite judgment. Shortcomings in the evidence may be any of the following: insufficient duration of studies, insufficient studies available (but evidence from at least two independent cohort studies or five case-control studies should be available), inadequate sample sizes, incomplete follow-up.
Limited, suggestive evidence: Evidence based mainly on findings from cross-sectional studies. Insufficient longitudinal observational studies or experimental studies are available or results are inconsistent. More well-designed studies of determinants are required to support the tentative associations.
Limited, no conclusive evidence: Evidence based on findings of a few studies which are suggestive, but are insufficient to establish an association between the variable and the behavioral outcome. No evidence is available from longitudinal observational or experimental studies. More well-designed studies of determinants are required to support the tentative associations.