What Is a Systematic Review?

If you’ve ever searched for research on a health question, you may have noticed that there are many studies to sift through. Some reach similar conclusions, while others seem to disagree. A systematic review is one way of making sense of all that research.

Rather than looking at one study in isolation, a systematic review brings together the available research on a clearly defined question. The researchers use a planned method to identify relevant studies, assess them and consider what they show when looked at as a whole.

This can be particularly useful when there are dozens, or sometimes hundreds, of papers on the same subject.

Why are systematic reviews used?

A single research paper can tell us something useful, but it rarely provides the full picture. The size of the study, the participants, the methods employed, or even chance may have influenced the results.

Imagine, for example, that several research teams have looked at whether a particular treatment helps with a health condition. One study finds a clear benefit. Another finds only a small difference. A third finds no meaningful effect.

Reading just one of those papers could provide you a rather different impression from reading all three.

A systematic review tries to bring this wider picture together. It asks a specific question and looks for the research that can help answer it. In a meta-analysis, researchers may also statistically combine the results from several studies where appropriate.

That does not mean a systematic review automatically provides a definite answer. The quality of the review is still dependent on the quality of the research available and the methods used by the review team.

How does a systematic review work?

Researchers generally start their search for studies with a plan. This matters because it reduces the chance of simply selecting papers that happen to support a particular conclusion.

The review question is normally defined cautiously. It might look at a treatment, a possible cause of a condition, a diagnostic test or another research question.

The researchers then decide where they will search for evidence. This can include databases about published research, as well as other sources where useful studies may be found.

They also set guidelines for inclusion of studies. For example, a review might only include studies of adults, or studies that tested a certain type of treatment. Other papers may be excluded by researchers as they do not meet the agreed criteria.

Once potentially relevant studies have been found, they are screened and assessed. Researchers look at things such as how the study was carried out, who took part and whether there are important weaknesses that could affect the findings.

The results are then brought together. Occasionally, the summary is mainly a written one. In other cases, statistical methods are used to combine numerical results from several studies.

What is the difference between a systematic review and an ordinary review?

The terminology can be confusing as the word “review” can mean a number of different things.

A traditional or narrative review describes research on a topic and explains the main ideas or findings. Getting an overview can be useful but the way studies are found and selected may not always follow the same structured process used in a systematic review.

A systematic review follows a more clearly defined method. The researchers describe their study search, selection criteria, and evidence assessment.

This makes the process easier for other researchers to examine and, in principle, repeat.

That does not make a systematic review completely free from judgement. Decisions must still be made about which studies fit the question, how to assess them, and how to handle differences.

What does a systematic review tell you?

A good systematic review can help show what the overall body of evidence suggests.

It may show that several studies point in the same direction. It might also reveal that the evidence is mixed or that there are not enough good-quality studies to reach a firm conclusion.

This last point is important. “There is not enough evidence” is not the same as “the treatment does not work.” Sometimes researchers simply do not have enough reliable information to tell.

A systematic review can also highlight gaps in existing research. If most available studies are small, poorly designed or focused on a particular group of people, the review may make it clear where further research is needed.

What is a meta-analysis?

You will often see the term ‘meta-analysis’ alongside ‘systematic review’, although the two are not exactly the same.

A meta-analysis is a statistical method for combining results from different studies. It can produce an overall estimate based on the available data.

Not every systematic review includes a meta-analysis. Studies may be too different from one another to combine sensibly. For example, they might use different treatments, measure different outcomes or involve very different groups of people.

In those situations, the review may describe and compare the findings without producing one combined numerical result.

Are all systematic reviews reliable?

Not necessarily.

The label “systematic review” does not, in itself, guarantee that the conclusions are strong. It is worth considering the conduct of the review and the research included.

One important question is whether the search was broad enough to find the relevant evidence. Another is whether the studies included were evaluated for possible biases or weaknesses.

You should also think about how many studies were included, and whether they generally agreed with each other. A review based on several good studies is likely to provide a different level of evidence to one based on a handful of small or unreliable studies.

The conclusions should also match the evidence. If the available research is uncertain, a careful review should make that uncertainty clear rather than presenting the findings as more definite than they are.

Why systematic reviews matter

Research can be difficult to interpret when you are looking at individual studies one at a time. Systematic reviews offer a way of stepping back and looking at a larger body of evidence.

They are widely used in health and medical research because they can help researchers, healthcare professionals and others understand what is already known about a particular question.

For readers, the main thing to remember is that a systematic review is not simply a collection of research papers. It is a structured attempt to identify, assess and bring together relevant evidence.

It can be a useful starting point when you want to understand the bigger picture, but it still needs to be read critically. The methods used, the quality of the included studies and the level of uncertainty all matter.

In the end, a systematic review is only as useful as the evidence and methods behind it. A well-conducted review can make a complicated area of research much easier to understand. It can also show where the evidence remains uncertain — which, in research, can be just as important as knowing what we already understand.