A defibrillator which assesses cardio pulmonary resuscitation carried out by a person on a subject and provides CPR feedback to the person, comprising: a biosignal measurement system configured to measure biosignals of the subject, determine when CPR is required and produce a CPR start signal and determine and when CPR is no longer required and produce a CPR stop signal, a CPR measurement system configured to measure compression signals during CPR chest compressions by the person, a CPR assessment system connected to the biosignal measurement system to receive the CPR start signal and the CPR stop signal, connected to the CPR measurement system receive compression signals and configured to perform the steps: (i) receive the CPR start signal and produce a first feedback signal, (ii) receive compression signals measured during a first plurality of chest compressions by the person, (iii) use the compression signals to establish a first CPR performance measurement of the person, (iv) set a CPR performance baseline of the person equal to the first CPR performance measurement of the person, (v) receive compression signals measured during a further plurality of chest compressions by the person, (vi) use the compression signals to establish a further CPR performance measurement of the person, (vii) compare the further CPR performance measurement of the person with the CPR performance baseline of the person, (viii) when the further CPR performance measurement of the person is greater than the CPR performance baseline of the person, produce a second feedback signal and set the CPR performance baseline of the person equal to the further CPR performance measurement of the person and go to step (x), (ix) when the further CPR performance measurement of the person is less than the CPR performance baseline of the person, produce a third feedback signal and go to step (x), (x) when the CPR stop signal is not received, return to step (v), (xi) when the CPR stop signal is received, produce a fourth feedback signal, and a feedback unit connected to the CPR assessment system and configured to receive the feedback signals and issue CPR feedback to the person. a biosignal measurement system configured to measure biosignals of the subject, determine when CPR is required and produce a CPR start signal and determine and when CPR is no longer required and produce a CPR stop signal, a CPR measurement system configured to measure compression signals during CPR chest compressions by the person, a CPR assessment system connected to the biosignal measurement system to receive the CPR start signal and the CPR stop signal, connected to the CPR measurement system receive compression signals and configured to perform the steps: (i) receive the CPR start signal and produce a first feedback signal, (ii) receive compression signals measured during a first plurality of chest compressions by the person, (iii) use the compression signals to establish a first CPR performance measurement of the person, (iv) set a CPR performance baseline of the person equal to the first CPR performance measurement of the person, (v) receive compression signals measured during a further plurality of chest compressions by the person, (vi) use the compression signals to establish a further CPR performance measurement of the person, (vii) compare the further CPR performance measurement of the person with the CPR performance baseline of the person, (viii) when the further CPR performance measurement of the person is greater than the CPR performance baseline of the person, produce a second feedback signal and set the CPR performance baseline of the person equal to the further CPR performance measurement of the person and go to step (x), (ix) when the further CPR performance measurement of the person is less than the CPR performance baseline of the person, produce a third feedback signal and go to step (x), (x) when the CPR stop signal is not received, return to step (v), (xi) when the CPR stop signal is received, produce a fourth feedback signal, and (i) receive the CPR start signal and produce a first feedback signal, (ii) receive compression signals measured during a first plurality of chest compressions by the person, (iii) use the compression signals to establish a first CPR performance measurement of the person, (iv) set a CPR performance baseline of the person equal to the first CPR performance measurement of the person, (v) receive compression signals measured during a further plurality of chest compressions by the person, (vi) use the compression signals to establish a further CPR performance measurement of the person, (vii) compare the further CPR performance measurement of the person with the CPR performance baseline of the person, (viii) when the further CPR performance measurement of the person is greater than the CPR performance baseline of the person, produce a second feedback signal and set the CPR performance baseline of the person equal to the further CPR performance measurement of the person and go to step (x), (ix) when the further CPR performance measurement of the person is less than the CPR performance baseline of the person, produce a third feedback signal and go to step (x), (x) when the CPR stop signal is not received, return to step (v), (xi) when the CPR stop signal is received, produce a fourth feedback signal, and a feedback unit connected to the CPR assessment system and configured to receive the feedback signals and issue CPR feedback to the person.