REVIEW ARTICLES

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SUMMARIZING RESEARCH FINDINGS: SYSTEMATIC REVIEW AND META-ANALYSIS

LP Wong, MSc, PhD
Lecturer, Health Research Development Unit (HeRDU), Faculty of Medicine, University of Malaya

Address for correspondence: Dr Wong Li Ping, Lecturer, Health Research Development Unit (HeRDU), Faculty of Medicine, University of Malaya, 50603  Kuala Lumpur, Malaysia. Tel: 603-79675738, Fax: 603-79675769, Email: wonglp@ummc.edu.my

THE NEED FOR REVIEWS
The huge amount of medical information available and its exponential growth have become common problem in the literature of biomedical information. Health care practitioners face the explosion in biomedical knowledge which makes keeping up with the primary research an impossible feat. There are approximately 17,000 new biomedical books published every year along with 30,000 biomedical journals, with an annual increase of 7%. For instance, MEDLINE alone contains more than eleven million citations, and more than 400,000 articles are added to the file each year.1 Majority health care providers noted that the current volume of scientific literature is unmanageable2 and often do not have sufficient time for reading medical journals as the information explosion continues.³ Further, some of these studies could be unclear, confusing or may also have contradicting results.

THE NARRATIVE REVIEWS

To make this task easier and manageable for health care providers as well as decision makers, often reviews are among their information resources. Reviews have always been a part of the medical literature. Traditionally, the medical research has been integrated in the narrative or nonsystematic form. An expert in a particular field will review studies, decided on the relevance, and highlight the findings, both in terms of results and, to a lesser degree the methodology.⁴ Such narrative reviews tend to be unsystematic and susceptible to many biases. Firstly, no systematic approach is prescribed to obtain the primary data and to integrate the data. Often, subjective judgment of the reviewer was used. There were often no explicit standards exist to assess the quality of review. Moreover, narrative reviewer also does not synthesize data quantitatively.

A CALL FOR SYSTEMATIC REVIEWS
A systematic review is defined by the Cochrane Handbook as ‘A review of a clearly formulated question that uses systematic and explicit methods to identify, select and critically appraise relevant research, and to collect and analyse data from the studies that are included in the review’. In contrast to narrative review, systematic review allows readers to appraise how the review was conducted and synthesized. It is of particular value in bringing together a number of separately conducted studies, sometimes with conflicting findings, and synthesizing their results. Systematic reviews have been proven to be able to yield valid, precise, and widely applicable answers to clinical questions.^5, In short, systematic reviews summarise large amounts of information and are more likely than individual trials to describe the true clinical effect of an intervention. Thus, systematic reviews have come to play a central role in informing clinical decisions and guidelines. A systematic review is also often called an ‘overview’.

A meta-analysis takes a systematic review one step further by statistically pooling the results of combinable studies. Since its introduction, meta-analysis has established itself as an influential branch of clinical epidemiology and health services research, with hundreds of meta-analyses published in the medical literature each year.⁶

THE PROCESS OF SYSTEMATIC REVIEW
Systematic review should be carefully planned with a detailed written protocol prepared in advance as any other search project. Systematic review involves several discrete steps and the steps are summarised below.

Step 1: Formulate review question. This requires the formulation of a clear statement of relevant patient groups, intervention of interest, as well as outcomes. The details are used to select studies for inclusion in the review.

Step 2: Locate studies. Systematic review must be undertaken in accordance with a predefined search strategy that would allow the completeness of the search to be assessed. Search strategies should consider the following sources: The Cochrane Controlled Trials Register (CCTR), other electronic databases and trials registered not covered by CCTR, checking reference lists, hand searching of key journals and personal communication with experts in the fields. The selection of primary studies is governed by inclusion and exclusion criteria that are initially specified when the protocol is defined.

Step 3: Appraising the quality of studies. After an exhaustive search, all possible primary studies that have been identified need to be assessed for eligibility for inclusion. Application of stringent inclusion/exclusion criteria should be addressed for example types of participants, interventions, outcomes, study designs and methodological quality. Independent assessment by more than one observer is desirable.

Step 4: Combining the results. The findings from combinable individual primary studies are then pooled to produce an ‘overall estimate’ on the clinical effectiveness of the intervention. The aggregation can be qualitative, or more appropriate, by statistically combining the data produced by individual studies into a single summary estimate. The statistical pooling of data is termed meta-analysis (Figure 1). In meta-analysis, results from studies are combined using ‘inverse variance method’, whereby larger studies and studies with less random variation are given greater weight than smaller studies.

Figure 1. Meta-analysis: Data from several different studies are combined and produce a single estimate

Meta-analysis can only be undertaken when studies address the same question, administer the intervention in a similar manner or measure the same outcomes. When studies differ in one or more of these components, meta-analysis is not appropriate. Therefore, systematic review may or may not include meta-analysis.