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Writing a Systematic Review Part I:
How to Design your Aim

Release Date: July 3, 2017
Category: Scientific Writing
Author: Katherine S., Ph.D., E.L.S.

So, you want to write a systematic review?

Well, I was thinking about it, but I’m not sure. It’s kind of daunting.

Systematic reviews are a lot of work, but they are also a valuable part of scientific literature. There are a lot of papers out there, and more are being published every day. Systematic reviews provide people a quick way to determine which papers they can trust and what those papers are telling us. They also allow us to get a big picture of what is known in a field.

Systematic reviews require a lot of planning, but once you design a solid study plan, it moves very quickly. A little extra footwork at the beginning will save you a lot of time down the line.

Where do I start?

First, you will need to figure out if someone is writing or has already written your review. If your review is going to cover a health-related topic, several repositories keep databases of existing and ongoing reviews. The best-known repository is the Cochrane Collaboration, which publishes health-related systematic reviews. There is also PROSPERO, the international prospective register of systematic reviews, and the Centre for Reviews and Dissemination Database, which has a searchable list of all proposed systematic reviews, including Cochrane proposals. Repositories also have a lot of great resources, including software, to help you through your journey.

Okay, my review topic is unique. How do I get started?

The next step is to put together your plan. You will need a clear picture of how to set up a systematic review and what things a reader will expect to see. Thankfully, someone has already done this work for you. A working group of international scientists, editors, and consumers put together standards for performing and reporting systematic analyses called the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). The information provided on the PRISMA site has everything you need to help you properly design a study, including checklists and detailed descriptions of what analyses you are expected to perform. Use the PRISMA statement as the foundation for building your review. The checklist will guide you on what pieces of information to include.

Got it. Now let’s do this!

Whoa there! There are some things you need to do before you jump in. First, what is your aim for the systematic review?

I’m looking at research on condition X.

That’s a little vague. As an experiment, type “condition X” into a PubMed search. How many hits did you get?

44,865 items.

Do you want to review 44,865 papers?

Hmmm, no.

Then you need a better aim. It is best if you can easily identify the information for which you are looking. It is also helpful if the aim is based around easily searchable terms so you can narrow down the pool of relevant articles. The Cochrane Collaboration suggests using the PICOS approach. PICOS stands for Population or Patient, Intervention, Comparison, Outcome, and Study Design.

“Population or patient” is the subset of people you are investigating. Are you looking at healthy adults, patients with condition X, or patients at risk for condition X? In addition, how will you define condition X? That definition can vary in the literature, and you need to decide up front which definition you want to use.

How about a review that focuses on adult patients with condition X, and we will use the definition of condition X from a seminal paper in the field?

Perfect. For the “Intervention,” you need to know what is being tested. Interventions can include things like drugs, devices, or procedures. Let’s say you are looking at adult patients with condition X. There are several ways to treat condition X, including treatments L, M, N, O, P, and combinations of treatments.

Let’s focus on papers related to treatment P alone.

Now we need to “Compare” the intervention with something. What do they compare it with in the literature? Placebos, standard therapy, some sort of “gold standard?”

Most of the studies compare the treatment to a placebo. I think we should go with that.

Next is the “Outcome.” Studies don’t always report the same thing. When you pick an outcome to look at, you want something that is clearly defined and experimentally validated. Details are important here. How was the treatment administered? What was the timing of the treatment? It doesn’t necessarily have to be the same in all studies, but it helps if you know what can be compared directly and what can’t.

In the field of condition X, a lot of trials use disease-free survival and how well people perform certain activities, such as tasks R and S, as their primary or secondary outcomes. Those things are really the goals of the treatment.

Those outcomes are relevant and seem pretty manageable. The last thing to consider is the “Study design.” As I’m sure you know, not all studies are created equal. Randomized controlled trials (RCTs) have the least bias and the highest strength of evidence. However, some topics don’t have a lot of RCTs available. You can still get information from less-stringent study designs, but you need to make the reader aware that the strength of this evidence may be lower.

There are a bunch of RCTs on this topic, so I think I’ll stick with those.

Let’s put this all together. What is your aim now?

I want to see if treatment P improves disease-free survival and the ability to perform tasks R and S compared with placebo in patients with condition X. Data will be the results of RCTs.

Now that you have an aim, you need to determine your inclusion and exclusion criteria. We’ll get into that in our next article, “Writing a Systematic Review Part II: Identifying literature—Inclusion and Exclusion Criteria .”

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