An interesting conversation via Evidence-Based Health. I've wondered how closely (and effectively) systematic review and clinical guideline searches are scrutinized during peer review. My guess is not very. But given their significant role in such projects, they certainly warrant careful review. Fortunately, as someone mentioned in the exchange, exceptional research has been done in this area:
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It's a great paper. The authors offer a number of recommendations that I think provide a nice framework for critically evaluating searches. Definitely worth a read.
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Edit: Also take a look at the checklist the authors put together based on their research. Another fine contribution.