I recently attended a course on evidence synthesis. It was very informative and useful so I thought why not write a blog about it. Now on the way back in train I am writing this blog.
So what is evidence synthesis. You may or may not be familiar with this term but I believe it is not that common yet and many researchers are still not familiar with it.
I do have a little bit of experience in this field but I got the opportunity to attend this training, so I thought better to improve my skills. What we usually do in our master’s or PhD thesis is that we write a general literature. So how is this literature review different than systematic ones.
To make it less boring I am going to discuss each type separately:
Lets start with the good old ‘literature review’:
Literature review:
What: This is usually first chapter of thesis. This chapter is named as ‘Introduction’ too. Literature reviews are also written as research articles and then published. But these can also be written as papers which are not necessarily part of a thesis. These are usually an overview of a specific topic starting broadly, identifying research gap and then narrowing down to the research question.
How: Research articles are searched, and selected papers are discussed and cited in it. There is usually critical discussion on the research papers as well. When writing a literature, we may have a bias when selecting papers for our literature review chapter/paper. We may select only those papers whose findings align with our own research topic. Even if there isn’t intentional bias, we may have unconscious bias while selecting papers. Moreover, usually grey literature Is not included. I will explain about grey literature in the below sections.
Why: It gives an overview of the research topic. It may help explain the research gap. Good for providing information about a topic all at one place, this way may help researchers to find information about their area of interest all at one place.
Evidence synthesis: Evidence synthesis is a type of research method through which researchers bring together all relevant information on a research question. This includes systematic review, systematic map, meta-analysis, rapid review and others. This can be useful to identify gaps in knowledge, establish an evidence base for best-practice guidance, or help inform policymakers and practitioners. Lets discuss the main types:
Systematic review:
What: As the name suggests, it is a literature review done in a systematic way. Initially, systematic reviews were done in the medical sciences. However, recently, environmental sciences are increasingly doing more systematic reviews.
How: In this type of review, we find an answer to a specific question based on the available evidence. For example, if someone is interested in finding out what is the effect of x on y. Say, what is the effect of a certain drug on a disease. For this question, if we were to do a general literature review, we would search and select published papers and we may have bias while selecting those papers. Secondly as mentioned earlier, in general literature review, grey literature is usually not included. What is grey literature. It includes thesis, reports from organisations, material from organisational websites. So if we do not include grey literature, there can be publication bias. What do we mean by that?? When a research project is done for example PhD research, usually only the good results or the positive results are published and the negative or not so promising results are not published and this leads to publication bias. The publications out there are usually the good and positive results of a project. The negative or unsuccessful results usually stay as a chapter only in a thesis and never sees the world of research publication. In this way by the end of our research we may find that most of the papers show positive effect of the drug on the disease, and we may conclude that this drug is very effective for treating that disease. This may lead to wrong decisions by the policy makers or stakeholders. That’s why we do systematic reviews in which we include all published research papers as well as grey literature to inform decision-making. Although, many negative or unsuccessful results are still somewhere sitting in someone’s computer or drawers, to which we have no access; however by doing a grey literature search, we may still be able to do a comprehensive search of literature by accessing theses and reports etc.
Why: The methods used for carrying out systematic reviews, help minimise bias. We minimise publication bias by including both published and grey literature. It helps provide all the information for a research question in a systematic manner.
Systematic map
Systematic maps are generally conducted in the same way as systematic reviews. There are some differences. It depends on the research question – we decide on the basis of research question whether it should be a systematic map or a systematic review. For example, questions such as what evidence exists relating to x. Say, what evidence exists on the impacts of xyz group of drugs on aquatic environment. Can you see the difference here? In a systematic review, the question is different, we are looking for what is the efficacy of a certain drug on a disease. Here in systematic map what we are doing is looking at what research is done relating to the impacts of drug on the environment. In this case we will gather all the articles that have looked at the different impacts, whether the impact is positive or negative or neutral, whatever. In this case a database, usually an Excel sheet is created with all the information which we can view in order of different categories.
A systematic review aims to synthesise the findings of multiple studies to answer specific research questions and provide evidence-based conclusions. In contrast, a systematic map categorises and visualises existing research on a broad topic to identify patterns, gaps, and the distribution of evidence without necessarily providing detailed synthesis or specific answers.
Meta analysis
What: It is used to combine and analyse data from multiple independent studies on a particular topic.
How: Meta analysis is done by gathering results from all the available material related to the research question, and then doing analysis where each study is a sample. For example, the research question is what is the effective dose of a drug for a certain age group. There will be different studies that had done research on finding the effective appropriate dose of a drug. The result from each study is used as a sample and then analysis is done. This will then potentially help to find an appropriate dose based on all the studies.
Why: Meta-analysis enables researchers to draw more robust and comprehensive conclusions about a given topic.
Evidence syntheses are time consuming and is usually done by a team, at least two people. Usually, softwares are used to conduct such reviews, which may require funds although free versions are also available. Moreover, Excel could also be used for this type of review, however using a software makes it easier compared to using only Excel.
I am not suggesting that we should be doing a systematic review for our thesis, specially when doing Masters when we have a limited time. Even during a PhD, usually there is not enough time to conduct a systematic review. The purpose of this blog is to introduce you to the world of evidence synthesis and it’s not a debate that why we should or should not be doing evidence synthesis. There are PhD researchers who as a part of their project have to conduct a systematic review or a systematic map depending on their research question. In this training, I met a PhD student who is doing a systematic map as part of their PhD project. This is essential for them if their actual practical/field research is dependent on this evidence synthesis. For example, if someone is to test wheat varieties in some field experiments, but needs to find which ones are the best for their outcome; they would do a systematic map. It really depends on what is the research question.
If you want me to do another blog on how to do a specific type of review in detail, let me know in the comments section.