ABSTRACT
Explicit controls are specic groups of patients in whom the progress of the condition has previously been observed and documented. In some instances a previous case series may be used as the comparator group. In others, the case records of suitable patients can be retrieved and used specically for the purpose of constructing the comparator group. e demonstration of the eects of the early antihypertensive agents in preventing renal failure in patients with accelerated (malignant) hypertension relied on explicit controls (see Table 3.2.1).1 Similarly, the benets of intravenous N-acetylcysteine in the treatment of paracetamol poisoning were shown by a comparison with previous patients attending the Edinburgh Poisons Unit.2
