Control arm and comparator (investigator's choice)
The group in a randomised trial that gets the existing standard treatment, against which the new drug is judged. Whether the control is the true current standard, an outdated one, or a menu of options chosen by each doctor ('investigator's choice') changes how much a positive result means.
The comparator should be the best available treatment at the time; trials using a weaker control (chemotherapy alone when chemo-immunotherapy was standard, or a suboptimal dose) draw regulatory and ODAC criticism and may not translate into practice. Investigator's or physician's choice from a list of chemotherapies is common in later-line trials (ASCENT, DESTINY-Breast04, TROPiCS-02) and is realistic but heterogeneous. Head-to-head trials of two new agents are rare and mostly sponsored by challengers (MARIPOSA vs osimertinib, HARMONi-2 ivonescimab vs pembrolizumab). Placebo or observation controls are used where no active standard exists, as in adjuvant and maintenance settings.