Literature reviews

What is a literature review?

A literature review (or narrative review) is a critique and

summary of a body of literature about a specific subject.

Its purpose is to set the scene or context for a topic and

provide the reader with background for understanding

current knowledge. It may also identify gaps or

inconsistencies in existing knowledge on a topic.

Although selective in the material it includes, selection is not as rigorous as for a systematic review.

Steps of the literature review

Define your research question

Plan your inclusion and exclusion criteria

Develop your search terms

Conduct your search

Select articles - apply inclusion and exclusion criteria

Critically appraise articles

Analyse and interpret results and present conclusions

Selecting a review topic

This can be the hardest part of the process. Try a pre-search

before developing a topic of interest into a full research question:

1. Start by searching Cinahl for a general topic e.g. leg ulcers

2. Search for the word study and link this back to your previous search to limit your results to primary research.

3. Examine the search results for recurring themes or aspects of interest e.g. honey or compression therapy. Use these to develop your topic into a more structured question.

What is a systematic review?

A systematic review is a one-stop summary of the evidence about a research question. It is basically a “study of studies” aiming to identify all relevant studies, appraise their quality, and synthesize and analyse the findings.

It takes a much more rigorous and organised approach than the traditional literature review and will often be conducted by a team of researchers rather than by an individual.

A systematic review of RCTs is considered to be at top of the evidence pyramid because so many studies are used which greatly reduces bias.

Hierarchy of evidenceIntervention/experimental studies

Strong

Weak

Systematic review of RCNs

Randomised controlled trials (RCNs)

Non-randomised controlled clinical trials

Observational studies

Systematic review of cohort studies

Cohort study

Case-control study

Cross-sectional survey

Case series

Case reports

Craig, J.V. and Smyth, R., 2012, p.148

Steps of the systematic review

Define your research question and develop inclusion criteria

Search for studies

Select studies and collect data

Assess risk of bias in included studies

Analyse data and undertake meta-analysis

Address reporting biases

Present results and summary of findings tables

Interpret results and draw conclusions

Searching for studies

The researchers undertake an organised search to find andcollect all studies relevant to the research question.

It is called a “systematic” review because the search method and selection is done systematically. This selection process is set out in the methods section to show how and why certain studies were included or not.

The researchers then decide which studies are of a sufficiently high quality to include in the review and again, this process should be transparent in the methods section.

Systematic searching

Searching should be systematic, rigorous and objective, following transparent criteria which are auditable and repeatable by others in order to eliminate bias.

Searching should also be comprehensive, covering a wide range of material including books, journals, web resources.

Sources should include online databases, individual e-journals and print journals in the Library (hand searching).

Additional sources might include grey literature e.g. DoH & NICE publications, policy documents etc.

Types of systematic reviews

Integrative studies use non-statistical techniques to integrate, evaluate and interpret the findings of qualitative research articles by finding patterns and themes.

Meta-analyses are quantitative and use statistical techniques such as SPSS to compare and combine the results of a body of studies. Results often take the form of a Forest Plot which presents studies side by side, enabling the reader to see the bigger picture and how the studies compare with each other. This helps to obtain a combined estimate of effect and enhances understanding by finding relationships and patterns.

Spotting bias in articles

Selection bias - how were participants selected? What sampling method was used e.g. random, convenient, purposive etc.

Attrition - did any participants drop out during the course of the study? This is particularly important in qualitative studies where numbers of participants are lower.

Publication bias - studies with positive results are more likely to be published than those with negative or inconclusive findings. It is also easier for established researchers to be published, so studies by unknown authors may never appear.

Cochrane reviews

Cochrane Reviews are the gold standard of systematic reviews and are published by the Cochrane Collaboration, an international body of researchers.

Unlike other systematic reviews, Cochrane Reviews are regularly updated as new evidence becomes available and have an added level of rigor with special techniques to identify bias. They are also unique in offering a plain language summary which clearly explains the results.

The Cochrane Collaboration has produced a very useful glossary of terms used in systematic reviews.

https://www.cochrane.org/sites/default/files/uploads/glossary.pdf