Final answer:
Controlled studies enrolling limited numbers of patients usually fall into either phase II or phase III clinical trials.
Step-by-step explanation:
In clinical trials, the trial phase from the subject population can be determined based on the number of patients enrolled. Controlled studies that enroll limited numbers of patients typically fall into the phase II or phase III clinical trial phases.
Phase II clinical trials involve a moderate number of patients and are conducted to assess the dose/exposure response and establish effective doses for phase III trials. Phase III trials, on the other hand, involve a larger number of patients, often several thousands, to evaluate the efficacy and safety of the compound.