What are the lowest levels of evidence in Evidence-Based Medicine?

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Case reports sit at the bottom of the evidence hierarchy because they describe one patient or a small number of patients without a comparison group. Without a control, it’s impossible to know whether observed outcomes are due to the intervention or other factors, so causality and generalizability are limited. They’re also highly susceptible to biases like selective reporting and confounding, and they don’t allow assessment of how well results would translate to broader populations. That said, they can flag rare or novel phenomena and spark hypotheses for more rigorous studies. In contrast, randomized controlled trials reduce bias through randomization and controls, and systematic reviews and meta-analyses synthesize data from multiple studies to provide stronger, more generalizable conclusions. Therefore, among the options given, case reports represent the lowest level of evidence.