A data monitoring committee (DMC) – sometimes called a data and safety monitoring board (DSMB) – is an independent group of experts who monitor patient safety and treatment efficacy data while a clinical trial is ongoing. Need for a DMC Many randomized clinical trials are double-blind – no one involved with the trial knows what treatment is to be given to each trial participant. Blinding includes the participant, their doctor, and even the study personnel at the company or organization sponsoring the trial. Blinding is breached and true assignments disclosed only after the trial database is finalized. Clinical trials may test an unknown procedure or may continue for years, and there is justifiable concern about enrolling participants and exposing them to an unproven treatment without ongoing oversight of the preliminary results. The DMC is a group (typically 3 to 7 members) who are independent of the entity conducting the trial. At least one DMC member will be a statistician. Clinicians knowledgeable about the disease indication should be represented, as well as clinicians knowledgeable in the fields of any major suspected safety effects. Ethicists or representatives from a patient advocacy group may be included, particularly for research involving vulnerable populations. The DMC will convene at predetermined intervals (depending on the type of study) to review unblinded results. The DMC has the power to recommend continuation or termination of the study based on the evaluation of these results. There are typically three reasons a DMC might recommend termination of the study: safety concerns, outstanding benefit, and futility. Safety concerns The primary mandate of the DMC is to protect patient safety. If adverse events of a particularly serious type are more common in the experimental arm compared to the control arm, then the DMC would have to strongly consider termination of the study. This evaluation has to be made in consideration of risk/benefit. In many cases, the experimental arm could cause serious adverse events (chemotherapy, for example), but the resulting improvement in survival outweighs these adverse events. Overwhelming benefit In the fortunate situation that the experimental arm is shown to be undeniably superior to the control arm, the DMC may recommend termination of the trial. This would allow the company sponsoring the trial to get regulatory approval earlier and to allow the superior treatment to get to the patient population earlier. There are cautions here, though. The statistical evidence needs to be very