It's common practice - indeed, it's widely recommended - that systematic reviewers search multiple databases in addition to alternative sources of data such as the grey literature to ensure that no relevant studies are left out of analysis. However, meta-research on whether this theory holds up in practice is mainly limited to examinations of recall - in other words, reporting how many potentially relevant studies are picked up by an abbreviated search method as opposed to one that's more extensive. What's missing from this body of research, write Ewald and colleagues in a newly published study, is that recall studies compare items retrieved in absolute terms without considering the final weight or importance of each individual study - variables which will ultimately affect the direction, magnitude, and precision of the resulting effect estimate. Since larger studies with more caché are likely to have the greatest impact on the final estimate and certainty of evidence - and these studies are more likely to be picked up in even an abbreviated search - the added value of utilizing more extensive search strategies on a meta-analysis is left unclear.
To examine the impact of the extensiveness of a search strategy on resulting findings and certainty of evidence, the authors randomly selected 60 Cochrane reviews from a range of disciplines for which certainty of evidence assessments and summaries of findings were available. Thirteen reviews did not report at least one binary outcome, leaving a total of 47 for analysis. They then replicated these reviews' search strategies in addition to conducting 14 abbreviated searches for each review (e.g., MEDLINE only), such as limiting to one database or a combination of just two or three (e.g., MEDLINE and Embase only). Finally, meta-analyses were replicated for each of these scenarios, leaving out studies that would not have been picked up in the various abbreviated search strategies.
Searching only one database led to a loss of at least one trial in half of the reviews, and a loss of two trials in one-quarter of them. As may be expected, the use of additional databases reduced the loss of information. Overall, however, the direction and significance of the resulting effect estimates remained unchanged in a majority of the cases, as shown in Figure 1 from the paper, below.
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The use of abbreviated searches did, however, introduce some amount of imprecision, typically increasing standard error by around 1.02 to 1.06-fold. The inclusion of multiple versus a single database did not clearly appear to improve precision compared to a comprehensive search.
The authors note that these findings are particularly applicable to authors of potential rapid reviews and guidelines, where a consideration of trade-offs between speed and thoroughness is of great importance. Rapid reviewers should be aware that limiting search strategy may change the direction of an effect estimate or render an effect estimate uncalculable in up to one in seven instances, but this should be weighed against the benefits of a quicker time to the dissemination of findings, especially during emergent health crises where time is of the essence.
Ewald, H., Klerings, I., Wagner, G., Heise, T.L., Dobrescu, A.I., Armijo-Olivo, S., ... & Hemkens, L.G. (2020). Abbreviated and comprehensive literature searches led to identical or very similar effect estimates: A meta-epidemiological study. J Clin Epidemiol 128:1-12.
Manuscript available from publisher's website here.