A defibrillator assesses cardio pulmonary resuscitation carried out on a subject and provides feedback. The defibrillator measures bio signals of the subject, determines when CPR is required and produces a start signal and determines and when CPR is no longer required and produces a stop signal. A measurement system measures compression signals during CPR chest compressions, a CPR assessment system receives the CPR start signal and the CPR stop signal and receives compression signals. The defibrillator uses the compression signals to establish a first CPR performance measurement, set a performance baseline of the person equal to the first CPR performance measurement of the person, compare the further CPR performance measurement of the person with the CPR performance baseline of the person, produce a feedback signal and set the CPR performance baseline of the person equal to the further CPR performance measurement of the person and return to earlier operations.