A ranking of research study types by how reliably they establish cause and effect — not all research is equally convincing.From highest to lowest: systematic reviews of RCTs > randomized controlled trials > cohort studies > case-control studies > case series > in vitro/animal studies. This hierarchy exists because each step down introduces more potential for confounding and bias. A compound supported only by in vitro data has not cleared the bar for human efficacy evidence, regardless of how compelling the mechanism. Peptidings applies this hierarchy explicitly — the evidence tier on every compound is an honest assignment within this framework.