What is a Flow Diagram?
The flow diagram (also called flowchart or flow chart) is typically the first figure in the results section of your systematic review.
This diagram shows, in a visual way, the reviewers’ process of finding published data on the topic and how they decided whether or not to include it in the review. The reader can see quickly and easily how many studies you screened, how many you included, and what exclusion criteria you used.
The four stages of a Flow Diagram
The work described in the flow diagram is divided into four stages:
- Identifying the articles
- Screening the articles
- Deciding on the eligibility of the studies
- Finalizing the list of studies to include in the systematic review
1. Identifying the articles
In the first stage, run the searches you designed through the abstract and citation databases you selected (e.g., PubMed, Scopus). Note how many records the search returned. You may also choose to add records you identified from other sources, such as Google Scholar or the reference lists from relevant articles.
⚠️ Caution: Each database has specific guidelines on how to search for keywords of interest and how to combine keywords for an effective search. Therefore, your search strategy may have to differ slightly between databases. For example, the search function in PubMed is very literal: if you search for the word “rat” in titles, you will not receive titles with the word “rats.”
After running the searches in the databases you selected and adding the records you identified from other sources, combine all the records returned from the searches into one citation management program. You can use DistillerSR, EndNote, Sciwheel, Mendeley, Zotero, or even Microsoft Excel.
Your next action is to remove duplicate records. In Excel, for example, click on the “Data” tab and select “Remove Duplicates.” You will be asked to specify which identifier (column heading) to sort by. It is better to sort by identifiers such as PubMed’s PMID or the article’s digital object identifier (DOI), because they are unique identifiers.
Two articles may have the same title, so if you delete duplicates by title, you may inadvertently lose an important and valid source. Note how many unique records you have left after removing duplicates.
2. Screening the articles
In the second stage, one of the investigators reads the title and abstract of each record to determine whether the article contains material that would be relevant or helpful to the systematic review.
This is a simple “yes/no” choice. If you determine that the article should be excluded, you must also note the reason for exclusion. Reasons to exclude an article are typically “review article with no original data,” “no control group,” “not relevant to the research question and outcomes,” “opinion piece,” or “wrong population/setting/intervention.” Note the number of articles you excluded and the number of articles under each reason for exclusion.
In some cases, two investigators perform the title and abstract screening. They do not divide the workload between them! Each investigator screens every title and abstract, and then their decisions are compared. If one decides to exclude an article that the other believes should be included, they can check the full text together to arrive at a mutual decision. They can also ask a third person (typically the project manager or principal investigator) to make a decision regarding the inclusion of the study.
The review articles you exclude may contain references to useful research studies that were not returned in your original searches. In that case, you can add those “extra” studies to the number of your “additional records.”
⚠️ Caution: Sometimes, there will be two reasons to exclude an article. Be careful to select the most appropriate reason. For example, in many flow diagrams, you will see the exclusion criterion “article not in English.” If your search returns an article in a language that you cannot read, ask yourself: is the article relevant to the topic of the systematic review? If yes, then it is correct to exclude with the reason “article not in [the languages you can read].” However, if the topic of the article is not relevant to your systematic review, then your reason for exclusion should be “not relevant to the research question and outcomes.”
3) Deciding on the eligibility of the studies
In the third stage, you take the articles remaining after the title and abstract screening and read their full texts, to determine whether these articles would help you to answer your research question.
This full-text screening is performed by two investigators. Each one reads the full text of all articles and makes an “include/exclude” decision. As in the title/abstract screening, in the full-text screening, you must note the number of articles you exclude and the number of articles under each reason for exclusion.
Again, disagreements between investigators regarding whether an article should be included or excluded are resolved by discussion or by asking a third investigator to read the article and make a decision.
4) Finalizing the list of studies to include in the systematic review
After excluding irrelevant studies in the full-text screen, you will know how many studies will be included in your systematic review. Note this number in your flow diagram.
In this fourth and last stage of screening, you determine how many of these studies can be included in a quantitative synthesis, also termed “meta-analysis.”
Not all studies that are eligible for the systematic review may be eligible for the meta-analysis. This is a statistical analysis that pools the data from multiple studies to test a hypothesis; not all studies will contain the data necessary for the quantitative synthesis. Note the number of studies for the meta-analysis in the last (bottom) box of the flow diagram.
Flow diagrams are easy to graph using software such as Microsoft PowerPoint, Excel, and Visio. You can also find free flow diagram generators online.To learn more about systematic literature reviews and how to conduct and report them, check out the self-study courses at Edanz Learning Lab.
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Dr Dean Meyer has a background in environmental science with a specialist interest in toxicology and public health. Her doctoral research work focused on molecular mechanisms of metal detoxification in an invertebrate model. Her other research interests include the mechanisms of toxicity and disease causation, and the occupational sources of xenobiotics and their physiological effects.
Dr Meyer spent 8 years working at the Centers for Disease Control and Prevention in Atlanta, and has an extensive background in the areas of laboratory safety and environmental health.
Dean is a certified Editor in the Life Sciences (ELS) and joined Edanz Group as an editor in 2015.