Understanding the Differences
Between a Systematic Review
vs Meta Analysis

The advent of evidence-based medicine has increased the demand for systematic methods to analyze and synthesize clinical evidence. When it comes to the search for the best available clinical evidence, randomized control trials, systematic reviews, and meta-analysis are considered the “gold standard” [1].

Since both systematic reviews and meta-analyses are secondary research approaches (research of research), sometimes the terms are used interchangeably, but there are vast differences between them.

A systematic review is a review that collects, critically appraises, and synthesizes all the available evidence to answer a specifically formulated research question.

A meta-analysis, on the other hand, is a statistical method that is used to pool results from various independent studies, to generate an overall estimate of the studied phenomenon.

Systematic reviews can sometimes use meta-analysis to synthesize their results, but they are two very distinct techniques. In this article, we will look at the definition of a systematic review, and understand how it is different from a meta-analysis.

What Is A Systematic Review?

In section 1.2.2 of the Cochrane Handbook, titled What is a systematic review?, the following definition can be found, “A systematic review attempts to collate all empirical evidence that fits the pre-specified eligibility criteria in order to answer a specific research question. It uses explicit, systematic methods that are selected with a view to minimizing bias, thus providing more reliable findings from which conclusions can be drawn and decisions made (Antman 1992, Oxman 1993). The key characteristics of a systematic review are: a clearly stated set of objectives with pre-defined eligibility criteria for the studies; an explicit, reproducible methodology; a systematic search that attempts to identify all the studies that would meet the eligibility criteria; an assessment of the validity of the findings of the included studies, for example through the assessment of the risk of bias; and a systematic presentation, and synthesis, of the characteristics and findings of the included studies”[2].

The evidence collected in a systematic review can be analyzed and synthesized, quantitatively, or qualitatively. The quantitative analysis of empirical evidence can use a meta-analysis as the statistical approach. To know more about how to write a systematic review, you can read our article; previously linked.

What Is Meta-Analysis?

Meta-analysis is a statistical method used to combine the results of individual studies. It uses a quantitative, formal, and epidemiological study design to systematically assess the results of previous studies to derive conclusions about a specific research parameter [3]. It is therefore an approach for systematically combining pertinent qualitative and quantitative study data from several included studies to establish a single conclusion that has significant statistical power.

Typically, the primary studies included in a meta-analysis are randomized controlled trials (RCTs). In a meta-analysis, the main objective is to provide more precise estimates of the effects of a treatment or of a risk factor for a disease, than any of the individual studies included in the pooled analysis. The data is also analyzed for heterogeneity (variation within outcomes), and generalizability (similarities between outcomes) within the individual studies, which facilitates more effective clinical decision making. Examining the heterogeneity of effect estimates within the primary studies is perhaps the most important task in a meta-analysis.

Meta-analyses of observational studies such as cohort studies are frequently performed, but no widely accepted guidance is available at the moment. While these meta-analyses are frequently published in literature, they are considered suboptimal to those involving RCTs.  The main reason is that the observational studies may entail an increased risk of biases and high levels of heterogeneity. Researchers who have to conduct meta-analyses on observational studies ought to carefully consider whether all included studies are able to answer the same clinical question.

Conclusion

Although meta-analysis is a subset of systematic reviews, a systematic review may or may not include a meta-analysis. An advantage of meta-analysis is that it has the ability to be completely objective in evaluating the research parameter. However, not all research areas have enough evidence to allow a meta-analysis. The inclusion of meta-analysis in a systematic review depends on the research question, the intervention to be studied, and the desired outcomes.

References

1. Sur RL, Dahm P. History of evidence-based medicine. Indian journal of urology: IJU: journal of the Urological Society of India. 2011;27(4):487–9.
2. Clarke M, Chalmers I. Discussion sections in reports of controlled trials published in general medical journals: islands in search of continents? Jama. 1998;280(3):280–2.
3. Haidich AB. Meta-analysis in medical research. Hippokratia. 2010;14(Suppl 1):29-37.