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USDA Nutrition Evidence Library Conclusion Statement Evaluation Criteria

Criteria for judging the strength of the body of evidence supporting the Conclusion Statement


Grade I: Strong

Grade II: Moderate

Grade III: Limited

Grade IV: Grade Not Assignable*

Quality (as determined using the RDI checklist)

·    Scientific rigor and validity

·    Consider study  design and execution

Studies of strong design


Free from design flaws, bias, and execution problems

Studies of strong design with minor methodological concerns

OR only studies of weaker study design for question

Studies of weak design for answering the question

OR inconclusive findings due to design flaws, bias, or execution problems

Serious design flaws, bias, or execution problems across the body of evidence


Consistency of findings across studies

Findings generally consistent in direction and size of effect or degree of association, and statistical significance with very minor exceptions

Some inconsistency in results across studies in direction and size of effect, degree of association, or statistical significance

Unexplained inconsistency among results from different studies


Independent variables and/or outcomes are too disparate to synthesize OR single small study unconfirmed by other studies


·    Number of studies

·    Number of subjects in studies


Several good quality studies

Large number of subjects studied

Studies have sufficiently large sample size for adequate statistical power

Several studies by independent investigators

Doubts about adequacy of sample size to avoid Type I and Type II error

Limited number of studies

Low number of subjects studied and/or

inadequate sample size within studies

Available studies do not directly answer the question OR no studies available


·    Directness of studied outcomes

·    Magnitude of effect


Studied outcome relates directly to the question

Size of effect is clinically meaningful


Some study outcomes relate to the question indirectly

Some doubt about the clinical significance of the effect


Most studied outcomes relate to the question indirectly

Size of effect is small or lacks clinical significance

Studied outcomes relate to the question indirectly

Size of effect cannot be determined

Generalizability to the U.S. population of interest

Studied population, intervention and outcomes are free from serious doubts about generalizability

Minor doubts about generalizability

Serious doubts about generalizability due to

narrow or different study population, intervention or outcomes studied

Highly unlikely that the studied population, intervention AND/OR outcomes are generalizable to the population of interest

Last Updated: 03/20/2014