Regardless of what methods of searching the literature you rely upon, the most important question is whether your search is adequate. You can think of the adequacy of your search in three ways. First, is the sample of studies you have obtained representative of the population of studies, or is it instead biased (as illustrated in Figure 3.2)? Second, does the sample of studies you have obtained provide sufficient statistical power to evaluate the hypotheses you are interested in (or, similarly, does it provide sufficiently narrow confidence intervals of effect size estimates to be useful)? Third, would the typical scholar in my field find the sample of studies complete, or have I missed studies that obviously should be included? The first two questions directly affect the quality of the empirical conclusions of your meta-analysis and so are obviously important. The third question is less important to the conclusions drawn, but is pragmatically relevant to others’ viewing of your review as adequate. This is a worthy consideration affecting both the likelihood of publication of your review and the impact it will have on your field.
The question of whether the sample you have obtained is an unbiased representation of the population is impossible to answer with certainty. However, there do exist methods of evaluating for the most likely bias—that of publication bias—which I describe in Chapter 11.
Probably the best way to answer all of these questions satisfactorily is to make every reasonable effort to ensure that your search is exhaustive—that is, to ensure that the sample of studies for your meta-analysis contains as close to all the studies that exist in the current population as possible. This goal is probably never entirely attainable, yet if you have made every effort to obtain all available studies, it is reasonable to conclude that you have come “close enough.”12 No one knows when “close enough” is adequate, and there is less empirical evidence to inform this decision than is desired, but I offer the following suggestions for your own consideration of this topic.
First, you should conduct an initial search using some combination of the methods described above that you expect will provide a reasonably thorough sample of studies. For example, you might decide to consult prior (narrative or meta-analytic) reviews in this area, search several electronic databases in which you believe relevant studies might exist (ensuring that these electronic searches include searches of unpublished studies such as dissertations), several listings of unpublished studies (i.e., conference programs, funding databases, and any research registries that exist in your field), and send out a request to authors via e-mail or listserv/website postings.
Second, you should create a list of studies obtained from these sources and ask some colleagues familiar with this research area to examine this list along with your inclusion/exclusion criteria. If they view it as complete, you have a good beginning. However, if they identify studies that are missing but should have been found, you should revise your search strategies (e.g., specifying different key words for electronic searches) and repeat the prior step.
The third suggestion is to take this list and begin forward and backward searches. You might start with forward searches, as this is less timeconsuming. Here, you would start with a small number of the most seminal works in the area (in the absence of a clear idea of the seminal works, you might create a short list of the first studies and the studies published in the top journals in your field). After performing forward searches with these seminal works (spending considerable time reviewing the citing papers to ensure relevance, as these types of searches are usually low in precision), you probably will have identified some additional studies; if not, you can reasonably conclude that forward searching will not yield any additional studies. Then, you can begin performing forward searches with the remaining studies, perhaps starting with the oldest studies first, as these have existed for the longest time and have therefore had more opportunity to be cited. At some point, you will likely reach a point where forward searches of more articles no longer yield new articles, and you can stop forward searching.
At this point, you can begin coding studies (see Chapters 4-7). While doing so, you should also perform backward searches (i.e., reading the works carefully for citations to other potentially relevant studies). My experience is that I often find a considerable number of additional studies when I begin coding, but that this number quickly diminishes as I progress in coding studies. If you find that you are almost never identifying additional studies near the end of your coding, you can be reasonably confident that your search is approaching exhaustion.
Despite this confidence, I recommend two additional steps to serve as a reality check. First, sit down with a few years of journals that are likely to publish studies relevant to your meta-analysis, and simply flip through the tables of contents and potentially relevant studies.13 If you do not find any additional articles, then this adds to your confidence that you have conducted an exhaustive search. However, if you do find additional articles, then you obviously need to revise your search procedures (if you find relevant articles, carefully consider why they were not found—e.g., did the authors use different key words or terminology than you used in your search?). The second step, if your flipping through the journals suggests the adequacy of your search, is to send the list of studies again to some experts in your field (preferably some who did not evaluate the initial list). If they identify studies you have missed, you should revise your search procedures; but if they do not, you can feel reasonably confident that your search is adequate.
My intention is not to be prescriptive in the process you should take in searching the literature. In fact, I think that the search process I described is more intensive than that used for most published meta-analyses. Nevertheless, I present these steps as a model of a process that I believe leaves little uncertainty that your search is “close enough” to exhaustive. Although there is no guarantee that you have obtained every study from the population, I believe that after taking these steps you have reached a point where more efforts are unlikely to identify additional studies and are therefore not worthwhile. I also believe that no other potential meta-analyst would be willing to engage in significantly greater efforts, so your search represents the best that is likely to be contributed to the field. Moreover, by consulting with experts in your field, you have ensured that your peers view the search as reasonable, which usually means that reviewers will have a favorable view during the review process, and readers will view it as adequate after it is disseminated. In sum, I believe that strategies similar to the one I have described can provide a high degree of confidence that your search is adequate.
Source: Card Noel A. (2015), Applied Meta-Analysis for Social Science Research, The Guilford Press; Annotated edition.
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