Observational studies suggest associations. Intervention trials test them.

This chapter examines the methodology of individual-level intervention trials - most notably the randomised controlled trial (RCT), often regarded as the gold standard for evaluating effectiveness.

We explore the core components of trial design:

* Clear definition of intervention and control

* Randomisation to minimise confounding

* Allocation concealment

* Blinding of participants and investigators

* Standardised outcome measurement

* Intention-to-treat analysis

Randomisation is the central methodological innovation. By distributing known and unknown confounders evenly between groups, it strengthens causal inference.

The chapter also discusses ethical considerations - informed consent, equipoise, and risk–benefit assessment - as well as practical challenges such as recruitment, adherence, and attrition.

Trial phases, pragmatic versus explanatory designs, and issues of generalisability are examined.

While powerful, individual trials are not without limitations. They can be costly, may not reflect real-world conditions, and sometimes exclude vulnerable populations.

Nonetheless, when rigorously designed, intervention trials provide the strongest evidence that changing exposure changes outcome.

They transform epidemiological reasoning into experimental proof.

Key Takeaways

* Randomisation reduces confounding.

* Allocation concealment and blinding minimise bias.

* Intention-to-treat preserves comparability.

* Ethical oversight is fundamental.

* Trials can be explanatory or pragmatic.

* Generalisability must be considered.

* Recruitment and adherence affect validity.

* RCTs provide strong causal evidence.

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