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2015 Dietary Guidelines Advisory Committee (DGAC) Nutrition Evidence Library Methodology

Literature Search, Screen, and Select Studies to Review

To minimize bias, inclusion and exclusion criteria were established for each SR question prior to searching the literature to guide the screening and selection process. Standard inclusion and exclusion criteria were established by the NEL and approved by the DGAC to promote consistency across reviews and to ensure that the evidence being considered in NEL SRs was most relevant to the U.S. population. These standard criteria were revised by the DGAC a priori as needed to ensure that they were appropriate for the specific SR being conducted. In general, criteria were established based on the analytical framework to ensure that each study included the appropriate population, intervention/exposure, comparator(s), and outcomes. They were typically established for the following study characteristics:
  • Study design
  • Date of publication
  • Publication language
  • Study setting
  • Study duration
  • Publication status (i.e., peer reviewed)
  • Type, age, and health status of study subjects
  • Size of study groups
  • Study dropout rate
Once inclusion and exclusion criteria were determined, they were used to guide the literature search process. Searching, screening, and selecting scientific literature was an iterative process that sought to objectively identify the most complete and relevant body of evidence to answer a SR question. The NEL librarians created and implemented search strategies that included identifying the most appropriate databases and search terms to answer each SR question. To optimize each search, NEL librarians peer review each other’s search strategies.
Existing high-quality literature reviews, including SRs and/or MAs, that addressed the topic or SR questions posed were identified by the NEL using a process called a duplication assessment. Existing SRs and MAs were valuable sources of evidence and were used for two main purposes in the NEL SR process:
  • To augment a NEL SR as an additional source of evidence, but not as an included study in the review (in this case, the studies in the existing SR or MA would not be included individually in the NEL review that was conducted); or
  • To replace a de novo NEL SR.
The NEL also used existing SRs to provide background and context for current reviews, inform SR methodology (e.g., inclusion and exclusion criteria, search strategy), and cross-check the literature search for completeness.

If relevant, low risk of bias, and timely existing SRs or MA were available, the reviews were compared and a decision was made as to whether an existing SR/MA would be used, or whether a de novo SR would be conducted. This decision was made based on the relevancy of the review in relation to the SR question and, when more than one review was identified, the consistency of the findings. If existing SRs/MA addressed different aspects of the outcome, more than one SR/MA may have been be used to replace a de novo SR.

Once the literature search was complete, the resulting articles were screened by two CNPP professionals independently, beginning with titles, followed by abstracts, and then full-text articles, to determine which articles met the criteria for inclusion in the review. Articles that met the inclusion criteria and relevant existing SR were hand searched in an effort to find additional pertinent articles not identified through the electronic search.

The DGAC provided direction throughout this process to ensure that the inclusion and exclusion criteria were applied appropriately and the final list of included articles was complete and captured all research available to answer a SR question. Each step of the process was documented to ensure transparency and reproducibility.

Related 2015 DGAC Methodology topics:

Last Updated: 02/15/2017