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Good review practice: a researcher guide to systematic review methodology in the sciences of food and health

Screening the results

Screening the results 

This step of the process is to screen and select relevant studies after searching is completed. Often it involves screening a large volume of studies and extracting appropriate data from them for inclusion in the review. The inclusion and exclusion criteria, the screening procedure and data extraction details are planned in the review protocol to guide this step. 

The screening procedure

The Screening Procedure 

The study selection process for inclusion in the review is done in the following steps: 

Step 1 Removing duplicates: searching various sources of information will result in duplicates due to the overlap of coverage between bibliographic sources and multiple records of same studies. To prevent double counting of the same studies duplicates should be identified and removed after the results of all searches are recorded and merged. If there is more than one record of the same study that needs to be included in the review, different versions should be clearly named and linked for verification.  

Step 2 Screening for relevance: the first level screening begins with titles and ‘titles and abstracts’ to ascertain the relevance of studies to the research question. Creating a screening checklist based on the key elements of the research question can facilitate this step. These questions can include the main characteristics of relevant populations or subjects, relevant interventions, relevant comparators, and appropriate timeframes of the studies etc.   When wrong elements are there, screen out.  If not, all elements are included, screen in as they might just not be included in title and abstract.  

Step 3 Screening for eligibility criteria: full texts of all studies passing for relevance at the first level should be obtained for screening against eligibility criteria. When there are uncertainties about the relevance of studies after ‘title and abstract’ screening, or they are judged as ‘potentially relevant’ they are also included for full text screening. 

A final list of the studies that meet every eligibility criterium set for the review is created and stored for quality evaluation.  An overview of the screening process and the steps involved are presented below. 

Figure 3 presents the study selection workflow in steps: removing duplicates; screening for relevance (screen 1); screening for eligibility criteria (screen 2). A final list of the studies that meet every eligibility criterium set for the review is created and stored for quality evaluation. 

Using screening tools:

Using Screening Tools 

Apart from reference management tools like EndNote, reviewers can consider using specialised screening tools for systematic reviews. A wide range of these tools have been developed in the last decade. They are built to accommodate different levels of processing and documentation. Some even use artificial learning technologies to automatically rank literature by relevance which make literature reviewing more time efficient than when they are only reviewed by humans. See Appendix B for tools.

Reporting the results of screening process:

Reporting the results of screening process

A flowchart can be used to report the number of included studies at each screening level. The number of excluded studies is expected to be recorded for each screening stage. It is important to record the reasons by which these studies were excluded according to inclusion/exclusion criteria. Often there are more than one reason for excluding some studies from the review. It is useful to organise the reasons in a way they can be tracked when there is a need for investigating the excluded studies later in the process. (See Appendix B for PRISMA frameworks)

Good Practice point: For transparency and reproducibility, expertise of the reviewers, how the screening decisions were made (e.g., independently or shared) and methods of resolving disagreements should be documented together with all other details of the process for future reference. Usually, at least two reviewers are expected to carry out the screening to reduce risk of reviewer’s bias. 
A note on terminology:  each result that is found in a database is called a record.  It may be a record of an article, a conference proceeding or some other document that reported in written form a study.   Multiple records might be found about a single study.  Each record in a database will be composed of a title, abstract and subject headings, so they are more than any of those parts, and should not be called references or citations.  For more on why this matters, read Record vs Report vs Study: Standard Terminology for the PRISMA Flow Diagrams in Systematic Reviews.