Welcome to the What are Evidence Syntheses section of the EBM Express course. 

 

EBM Express is designed to build your understanding of evidence-based practice in short manageable blocks of content. In this section, we’ll be looking more closely at what we mean when we talk about evidence syntheses, why a systematic review is not the same as a literature review, and what methodological components make up the most common types of evidence syntheses research studies. 

 

Simply put, evidence syntheses combine the results of multiple studies or sources of information into one evidence-based resource. 

 

Evidence syntheses come in many forms including point of care tools, decision tools, practice guidelines, and consensus development reports, and they also include grey literature, such as government reports and other syntheses published outside of the traditional publishing industry. Additionally, systematic reviews, meta-analyses, scoping reviews, and rapid reviews 

are types of evidence syntheses. These syntheses are all also types of research studies with specific methodologies and are the types of evidence syntheses that we’ll be focusing on in this section. 

 

But first, a note about the term review. There are lots of review articles which are sometimes referred to as literature reviews or narrative reviews. These typically involve an expert that reads much of the current literature on a topic and writes a summary of the topic. These can be helpful, but they are not a type of research study and do not follow a specific methodology. Because literature reviews do not follow a specific methodology, they are vulnerable to the bias of the expert author, and they are not considered evidence syntheses in the context of evidence based medicine. 

 

So, what is a systematic review? A systematic review is the most common evidence synthesis that is also a research study type. 

 

Systematic reviews are based on a clearly articulated, focused question. They will include very detailed information about the methods used to select articles that are included in the study. This means that the authors will share their search strategy and  

terms. They will list the databases that they searched, and it will be more than one. And they will tell you what inclusion or exclusion criteria that they imposed on the search set. This might include eliminating articles published before a certain date or may be based on patient age-groups or something more specific to their question. General, or literature, reviews are not usually asking a focused question but are more likely to be a topical overview. They might give you a brief statement that says how they found the articles they read to write the review, but it will not be very specific. Systematic reviews will also critically appraise the articles that they select so that readers can know how strong the evidence is. The evidence is then synthesized as much as possible given the differences in the unique studies that were looked at.  In a literature review, there is generally no discussion of the critical appraisal. Readers are relying on the expert’s decision to include information. These types of reviews are also not focused on synthesizing the evidence so much as they are writing a summary 

 

Let’s look at some examples, both from the Journal of Neurology and both labeled  

Review: 

 

First, “Channelopathies in epilepsy: an overview of clinical presentations, pathogenic 

mechanisms, and therapeutic insights.” This is the first page of the article which is 

from the Journal of Neurology, and it is labeled a Review. But you’ll notice that both in the title and in the Introduction, the authors refer to it as an overview.  As we scroll through the article, you will notice that although the article has lots of intext citations, there is no methods section to explain how the articles were found. The text is simply broken into topical sections that provide an overview of different channels and channelopathies. 

 

Now, let’s open “Mortality in patients with epilepsy: a systematic review”. This article 

is also from the Journal of Neurology and is labeled a review. However, this time the authors have clearly labeled it as a systematic review in their title and in the abstract. In the introduction, they state that they “will investigate mortality disparities within different subgroups with a focus on risk profiles influenced by geographic, gender, and age-related factors.” Although not formatted as a question, this is their focused question. 

 

As we scroll through this article, we see a methods section that details the specific search strategy, the databases searched, and the inclusion/exclusion criteria. They also include a quality assessment and accuracy of data section that tells us how they critically appraised the article. And sections that explain how they analyzed and synthesized the articles.  Also of note in this article is a discussion of searching for and incorporating grey literature as well as a flowchart that shows the steps taken in selecting the articles that were included. 

 

By comparing these two different reviews, it is easy to understand that a general literature review and a systematic review are distinctly different. A systematic review is considered a type of research study with rigorous and repeatable methods, while a literature review is not. 

 

Now let’s look at some of the other types of Evidence Syntheses that are also considered research studies. First, meta-analyses. 

 

A meta-analysis has all the same qualities of a systematic review: focused question,  detailed search methodology, critical appraisal, and synthesis of articles. But a meta-analysis takes the synthesis further by using statistical techniques to combine the  quantitative data from the individual articles. The ability to perform this combined quantitative analysis is dependent on the data within the articles and generally requires the skills of a statistician. A meta-analysis is a high level of evidence and is usually clearly labeled as a meta-analysis. These articles will include statistical methodologies and detailed charts to explain the analysis. 

 

Another type of evidence synthesis, is a scoping review. 

 

A scoping review is a preliminary assessment of the potential size and scope of available research literature on a topic. The goal is to identify the nature and extent of research evidence, including ongoing research. This type of review will be based on a research question. The methods will be transparent and detailed, explaining how articles were found and included, and therefore, are also repeatable.  

A scoping review is really an attempt to understand the ‘scope’ of the available literature and research related to the question. A scoping review does not typically include critical appraisal or true synthesis but does attempt to provide a narrative that gives some information about the quality and quantity of the available evidence. Frequently, a scoping review will recommend an appropriate study type for further  

analysis. 

 

A rapid review is time-sensitive. 

 

Rapid reviews are typically based on a current or emerging question. They attempt to be comprehensive and critically appraise the articles; however, one goal of this type of study is to make evidence available quickly so compromises may be made in regard to comprehensiveness and/or the critical appraisal. The methodology will still be fully explained and repeatable and any compromises that were made for timeliness should be detailed. 

 

To summarize: Evidence Syntheses combine the results of multiple studies or sources of 

information into one evidence-based resource. Not all reviews are created equally. Systematic Reviews, Meta-analyses, Scoping Reviews, and Rapid Reviews are all types of Evidence Syntheses that are also types of research studies with distinct and replicable methods. 

 

These are the references used in creating this section.