Studies have the least bias and results are considered valid. These studies adhere mostly to the commonly held concepts of high quality, including the following: a formal study design; clear description of the population, setting, interventions, and comparison groups; appropriate measurement of outcomes; appropriate statistical and analytical methods and reporting; no reporting errors; less than 20 percent dropout; clear reporting of dropouts; and no obvious bias. Studies must provide valid estimation of nutrient exposure from dietary assessments and/or biomarkers with reasonable ranges of measurement errors and justifications for approaches to control for confounding in their design and analyses.

Studies are susceptible to some bias, but not sufficient to invalidate the results. They do not meet all the criteria in category “A”; they have some deficiencies, but none likely to cause major bias. The study may be missing information, making it difficult to assess limitations and potential problems.

Studies have significant bias that may invalidate the results. These studies have serious errors in design, analysis, or reporting; there are large amounts of missing information or discrepancies in reporting.