The flow diagram (also called flowchart or flow chart) is typically the first figure in the results section of your systematic review. A PRISMA flow diagram visually depicts the reviewers’ process of finding published data on the topic and how they decided whether to include it in the review.
From a PRISMA diagram, the reader can quickly and easily see:
- how many studies the review screened
- how many were included
- what exclusion criteria were used
Understanding how to use and apply these flow charts is key in helping others help themselves by reading your review.
The 4 stages of a PRISMA flow diagram
The work described in the flow diagram is divided into 4 stages:
- Identifying the articles for review
- Screening the articles for review
- Deciding on the studies’ eligibility
- Finalizing the list of studies to include in the systematic review
1. Identifying the articles
You might also 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. This means 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 won’t receive titles with the word “rats”; you’ll only get “rat”.
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. For this, you can use:
Next, remove duplicate records. In Excel, for example, click on the “Data” tab and select “Remove Duplicates.” You’ll be asked to specify which identifier (column heading) to sort by.
It’s 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 accidentally 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 investigator reads the title and abstract of each record. They’re looking 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 are typically:
- “review article with no original data”
- “no control group”
- “not relevant to the research question and outcomes”
- “opinion piece”
- “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 this is the project manager or principal investigator) to make a decision on including 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’ll see the exclusion criterion “article not in English.” If your search returns an article in a language that you can’t read, ask yourself: Is the article relevant to the topic of the systematic review? If yes, then it’s 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 studies’ eligibility
In the third stage, you take the articles remaining after the title and abstract screening and read their full texts. This is to determine whether these articles would help you to answer your research question.
Two investigators perform the full-text screening. 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/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’ll 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 called a 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.
Go with the flow (chart): Here’s where to learn more
Flow diagrams are easy to graph using software such as Microsoft PowerPoint, Excel, and Visio. You can also find free flow diagram generators online.
Log in or Create a Free Account to view this interactive dissection of a real systematic review, including flow charts
Dr. Dean Meyer is a Board-certified Editor in the Life Sciences (ELS). She 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.