Literature reviews play a vital role in fulfilling the primary goal of any evidence-based research by aiding in decision-making with transparent and impacting results. With advances in all medical fields and with increased access to medical literature, different methodologies have been conceptualized for performing literature reviews, leading to the advent of different types of literature reviews. The main types of systematic review of literature include systematic literature reviews (SLRs) and its subtypes such as targeted literature reviews (TLRs), scoping reviews (ScR), and rapid reviews (RR); meta-analyses (MA) and its subtypes such as indirect treatment comparison (ITC), network meta-analysis (NMA), and individual patient data MA (IPD-MA).[1,2] While the expanded range of approaches have the potential to address broader types of evidence designs to cater to the increasing complexity of healthcare, the challenges in choosing the best review approach to meet the purpose of the review should also be acknowledged.[1,2] Often a new reviewer (and sometimes even the seasoned one) is confused about which review methodology is best suited for addressing the research question s/he has.
Systematic literature reviews are based on explicit, reproducible methods to search all sources of evidence and critically appraise a highly focused clinical question. SLRs are performed to compare two different interventions, to confirm relevant evidence, to assess the quality of evidence, and to address any variation in practice. SLRs generally involve two independent reviewers for screening, data extraction, and quality assessment, and are time and resource-intensive.[2,4,5]
Scoping reviews are performed to identify, map, and synthesize the available research if it is not yet comprehensively reviewed. They identify key characteristics or factors and usually act as a precursor to an SLR for identifying and analyzing knowledge gaps. ScRs may require larger teams because of the volume of literature.[2,4,6]
Rapid reviews are rigorous in their methodology, but set limits on the process for shortening the timeframe of review completion. They are mostly used for emerging, critical research topics, updates of previous reviews. However, curtailing the time for research may introduce inconsistencies and biases.[1,4,7]
Targeted/ focused literature reviews address clearly formulated questions by using explicit methods to identify, critically appraise, and qualitatively analyze key relevant research, and are usually performed by a single reviewer. However, TLRs are often not as comprehensive as SLRs, and may be less reproducible than an SLR.[4,8]
Meta-analysis is a systematic statistical procedure for combining data from multiple studies and then quantitatively synthesizing them. Results are often depicted graphically in the form of forest plots, and specialized statistical methods and software are often used. They are more time and resource-intensive than other forms of SLRs.[1,9]
Individual participant data (IPD) meta-analysis uses individual participant data from studies and performs statistical analyses to generate powerful and uniformly consistent results. The quality of data from IPD-MAs are quite high and the results are highly reliable, but the statistical methods of IPD-MA require specialized software and highly qualified statisticians. IPD-MAs are more lengthier and costlier than traditional MAs.[10,11]
Network meta-analysis is an umbrella term to describe indirect treatment comparisons (ITC) and mixed treatment comparisons (MTC). It extends the principles of meta-analysis by comparing multiple treatments simultaneously in a single analysis by combining direct and indirect evidence within a network of randomized controlled trials. The methodological approaches are complex, and the results are displayed graphically in the form of network diagrams.
To conclude, selecting the appropriate literature review approach largely depends on the research question, availability of resources, cost, time availability, and the intention of the research.
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