Journal club epidemiology critique — teaching/CASC-style station

You are the psychiatry registrar facilitating a 10-minute teaching station with a foundation doctor and a medical student. Station material is a one-page abstract: 'Cross-sectional household survey (n=1,200, response rate 48%). Lifetime depression by self-report checklist 28%. Cannabis use associated with depression OR 1.9 (95% CI 1.2–3.0). Authors conclude cannabis causes nearly one-third of depression (PAF 30%) and recommend universal school cannabis screening using a 2-item tool (Sn 80%, Sp 80%) to prevent depression.' Your job is to teach correct measure choice, bias threats, PAF assumptions, and why universal screening PPV will fail — without humiliating juniors or dismissing all observational evidence.