Stephen P. Glasser, MD, is professor of preventive medicine

Pharmacologic inhibition of the renin-angiotensin aldosterone system (RAAS) has become a widely accepted ap­proach to lowering blood pressure (BP).

A patient's response to drugs may be influenced by many factors, including age, race, sex, ethnic background, metabolic phenotype, body fat content and distribution, and body size. Drug–drug and drug–disease interactions are also important.

The previous installment of this series on clinical research highlighted some im­portant fundamentals in cohort studies and randomized controlled trials (RCTs). In this article, additional issues that are critical to the understanding of RCTs are discussed, including causal inference, the use of surrogate measures, and the principle of intention to treat (ITT).