A computer-implemented method of assessing concentration of a subject, and system and computer therefore, comprises tracking of the subject's eye gaze S10 when reading an assessment text to give one or more numerical results S40 based on either (i) a total return distance S20 provided by calculating the distance by which the subject's eye gaze has returned to a previous word in the assessment text each time the subject's eye gaze has returned to a previous position, to form a return distance, and by summing any return distances to reach the total return distance and/or (ii) and a return rate S30 provided by dividing a number of times the subjects eye gaze has returned to a previous position by a time taken to read the assessment text； before outputting the one or more numerical results S50 to assess the subject's concentration. The results of the method can be used in diagnosis of a mental health condition such as attention deficit hyperactivity disorder (ADHD) or depression.
