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Distribution of the types of primary studies in the included systematic reviews (n = 301). Quasi-experimental studies including non-randomized trials and before and after studies; Case-control studies including nested case-control studies, case-cohort studies and case-crossover studies; “Unclear” means that it is not possible to identify the specific type of study, such as descriptions studies, observational studies, prospective studies, and retrospective studies and so on.
