Explosion in the number of papers published in medical journals makes it difficult to keep pace with the primary research. Experts synthesize this accumulating knowledge in a summary for the benefit of a busy clinician. We find these reviews in all medical journals.
This traditional method of reviewing literature, commonly known as narrative review, has several disadvantages. One obvious problem is that, reviewers rarely begin with an open mind and review can be biased by their professional opinions. Further, reviewers may include only those studies that agree with their own opinions and may completely ignore studies that have reached to a different opinion.
A better way of reviewing medical literature is to develop a search strategy in order to identify all the relevant clinical trials and systematically review all these trials. Such a review is generally known as a ‘Systematic Review’. This approach should eliminate the bias resulting from selective inclusion of studies. Although a systematic review is better than a narrative review, it also has one major limitation. A systematic review is, generally, unable to reach to a conclusion without ignoring sample size, effect size, and research design of the clinical trial.
The limitation of systematic review can be dealt by statistically combining results of all relevant clinical trials. This method of reviewing literature is known as ‘Meta-analysis’. Such a review not only evaluates all the available literature on a particular topic, but also provides a summary estimate of the effect size after taking into consideration sample size, effect size and study design.
What is a Meta-Analysis?
Meta-analysis is a statistical procedure for combining data from multiple studies. When the treatment effect (or effect size) is consistent from one study to the next, meta-analysis can be used to identify this common effect with more precision than individual studies. When the effect varies from one study to the next, methods of meta-analysis can be extended to identify the reason for the variation.
Why do a Meta-Analysis?
Even a casual reader of medical literature will notice that results of clinical studies vary from one study to another. This variation in study results may not due to some problem with the study design or conduct, but rather can be due to pure chance alone. Thus, decisions about the utility of a treatment or the validity of a hypothesis cannot be based on the results of a single study. Now if we need multiple studies to identify the real effect (benefit or harm) of the treatment, we do need a mechanism to synthesize data across such studies. Narrative reviews are largely subjective, in contrast, meta-analysis applies objective formulas (much as one would apply statistics to data within a single study), to combine the results of any number of studies.
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