Systematic literature reviews

Methods for reviewing and evaluating the scientific literature range from highly formal, quantitative information syntheses to subjective summaries of observational data. The purpose of a systematic literature review is to evaluate and interpret all available research evidence relevant to a particular question. In this approach a concerted attempt is made to identify all relevant primary research, a standardized appraisal of study quality is made and the studies of acceptable quality are systematically (and sometimes quantitatively) synthesized. This differs from a traditional review in which previous work is described but not systematically identified, assessed for quality and synthesized.

Advantages

There are two major advantages of systematic reviews (or meta-analyses). Firstly, by combining data they improve the ability to study the consistency of results (that is, they give increased power). This is because many individual studies are too small to detect modest but important effects (that is, they have insufficient power). Combining all the studies that have attempted to answer the same question considerably improves the statistical power.

Secondly, similar effects across a wide variety of settings and designs provide evidence of robustness and transferability of the results to other settings. If the studies are inconsistent between settings, then the sources of variation can be examined.

Thus, while some people see the mixing of ‘apples and oranges’ as a problem of systematic reviews, it can be a distinct advantage because of its ability to enhance the generalizability and transferability of data.

Disadvantages

Without due care, however, the improved power can also be a disadvantage. It allows the detection of small biases as well as small effects.