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Nutrition Evidence Library Systematic Review Methodology

Literature Search, Screening and Selection

Searching, screening, and selecting scientific literature is an iterative process that seeks to identify the most complete and relevant body of evidence to answer a SR question. This process is guided by inclusion and exclusion criteria that are determined a priori. The NEL Librarians, in collaboration with the NEL Lead and/or Secondary Lead, create and implement a search strategy that includes a list of appropriate databases and search terms to identify literature to answer the SR question. Search strategies are co-peer reviewed by the librarians.  The results of the literature search are screened by the librarians and NEL staff in a dual, step-wise manner, beginning with titles, followed by abstracts, and then full text articles, to determine which articles meet the criteria for inclusion in the review. The reference lists of all articles that meet the inclusion criteria and related SRs are searched in an effort to find additional pertinent articles not identified through the electronic search. In addition, as part of this process, duplication assessment is conducted by NEL staff and the TEC to determine whether there are existing high-quality SRs or meta-analyses (MAs) that can be used to augment or replace a NEL SR.
The TEC provides input throughout this process, to ensure that the inclusion and exclusion criteria are applied appropriately and the final list of included articles is complete. Communication between the NEL staff and NEL Librarians, along with input from the TEC and stakeholders, is key for ensuring that each search conducted captures all research available to answer a SR question. In addition, each step of the process is meticulously documented to ensure transparency and reproducibility.
A Closer Look: Inclusion and Exclusion Criteria
Inclusion and exclusion criteria are established a priori for each SR question to guide literature screening and selection to ensure the most relevant and appropriate body of evidence is identified to answer the question. These objective criteria ensure the credibility and utility of the results of the SR to inform Federal policy and programs. NEL has established standard inclusion and exclusion criteria to promote consistency across reviews and ensure that the evidence being considered in NEL SRs is most relevant to the U.S. population. Criteria are established to ensure that each study includes the appropriate intervention/exposure, comparator(s), and outcomes, based on the analytical framework. In addition, criteria are typically established for the following types of study characteristics:
  • Study design
  • Date of publication
  • Publication language
  • Study setting
  • Study duration
  • Publication status (e.g., peer reviewed)
  • Type, age, and health status of study subjects
  • Size of study groups
  • Study dropout rate
A Closer Look: Conducting a Duplication Assessment
SRs are time and labor intensive to conduct, so the next step in the NEL process is to conduct a duplication assessment to determine whether there are any existing high-quality SRs and/or MAs that already address the topic or SR questions posed. Existing SRs and MAs can serve as valuable sources of evidence and may be used for two main purposes in the NEL SR process:
  • To augment a NEL review as an additional source of evidence to consider, but not as an included study in the review (in this case, the studies in the SR or MA would not be included individually in the NEL review that is conducted).
  • To replace a de novo review.
NEL also uses existing SRs to provide background and context for current reviews, inform SR methodology (e.g., inclusion and exclusion criteria, search strategy), and serve as a way to cross-check the literature search for completeness.
There are several important considerations that are made when determining eligibility for inclusion, such as: (1) relevance to the SR question of interest, (2) methodological quality, (3) timeliness, and (4) reference overlap. These considerations are described in more detail below:
  • Only SRs and MAs that are highly relevant to the question of interest will be eligible. Relevance will be determined based on a comparison of the proposed NEL SR question pre-specified PICO (population, intervention/exposure, comparator, outcomes) structure, analytic framework, and inclusion/exclusion criteria with that of the existing SR or MA.
  • Only SRs and MAs determined to be of high quality will be eligible to augment or replace a NEL SR. Methodological quality of SRs or MAs will be evaluated by NEL staff using the AMSTAR (Assessment of Multiple Systematic Reviews) [1] tool. Based on the AMSTAR scores, only those SRs or MAs that score 8 or higher (out of a possible 11 points; each “Yes” response received 1 point) are considered to be of high quality, and therefore, eligible for consideration.[2]
  • SRs and MAs are considered timely if published within the date range of the NEL review, but included studies may go beyond the search date window. If methodological considerations make older studies less relevant, the SR or MA may be excluded.
  • SRs and MAs will be reviewed for reference overlap. If overlap exists, the NEL staff will work with the TEC to determine how best to address the overlap, which may involve excluding some reviews of cross-over is substantial, or if the review of references identifies gaps in the literature search conducted.
If multiple relevant, high-quality, and timely SRs or MAs are eligible to answer the SR question, the conclusions of the reviews identified will be compared and a decision will be made whether the existing SRs/MAs identified will be used, or whether a de novo SR will be conducted.
[1] Shea BJ, Grimshaw JM, Wells GA, Boers M, Andersson N, Hamel C, Porter AC, Tugwell P, Moher D, Bouter LM. Development of AMSTAR: a measurement tool to assess the methodological quality of systematic reviews. BMC Medical Research Methodology. 2007, 7:10.

[2] Sharif MO, Janjua-Sharif FN, Ali H, Ahmed F. Systematic reviews explained: AMSTAR-how to tell the good from the bad and the ugly. Oral Health Dent Manag.2013 Mar;12(1):9-16. Erratum in: Oral Health Dent Manag. 2013 Jun;12(2):119.Sharif, Fyeza N Janjua [corrected to Janjua-Sharif, Fyeza N]. PubMed PMID:23474576.

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