Research design and critical appraisal

The hierarchy, and its limits

Systematic reviews and meta-analyses sit above randomised controlled trials, which sit above cohort and case-control studies, then cross-sectional studies, then case series. The hierarchy concerns causal inference specifically, and a well-conducted qualitative study answers questions an RCT cannot.

Key features of a trial

• Randomisation — controls for known and unknown confounders alike, which is what nothing else does.
• Allocation concealment — prevents the person recruiting from influencing which arm someone enters.
• Blinding — single, double or triple. Often impossible in psychological therapy trials, which is a real and acknowledged limitation.
• Intention-to-treat analysis — analyse participants in the arm they were randomised to, regardless of what they actually received. Per-protocol analysis reintroduces selection bias.
• Control condition — waiting list, treatment as usual, or active comparator. Waiting list controls tend to overstate effects.

Qualitative approaches

Clinical psychology uses qualitative methods routinely, and selection may ask about them. Thematic analysis identifies patterns across accounts. Interpretative phenomenological analysis examines how individuals make sense of experience, with small homogeneous samples. Grounded theory builds theory from data, sampling until saturation.

Quality here is judged by credibility, reflexivity, transparency of analysis and the fit between question and method — not by sample size or generalisability.

Where marks get lost

• Inferring causation from a cross-sectional correlation.
• Treating a per-protocol result as equivalent to intention-to-treat.
• Dismissing qualitative work for lacking generalisability, which is not what it is for.
• Ignoring that a waiting-list control inflates apparent effect size relative to an active comparator.