The NRC 'stressalyser' is a step-input subject-paced pursuit tracking task used to detect the effects of various extrinsic factors on skilled performance. It gives measures of reaction time, movement time, error score and overshoot score, each at four levels of the relevant within-task variable (directional probability, target distance or boundary distance). Several investigations have shown how these measures vary with such factors as sleep loss, time of day, alcohol, insulin-induced hypoglycaemia and diazepam, and thus demonstrate the applicability of the device to many applied problems of performance impairment.