The Analysis of Cardiac Interbeat Interval Sequences and the Effects of Mental Work Load

however, which must be watched. An obvious one is that the task used for measurement must be very carefully chosen to avoid interference of a simple mechanical kind with the main task; hence one uses spoken questions and answers for a driving task, whereas one might use visual presentation and written answers if one was studying the demands of an auditory task. Less obvious is the possibility that the need to carry out a second task may change the performance of the first. A clear instance of this is shown up, in a driving situation, by studies of the use of a radio-telephone while driving. Even when one excludes the mechanical factor of holding the telephone handset, it is possible to show that listening and speaking while driving do produce slight changes in the driving itself. If we set up an experimental situation, requiring a man to drive through very small gaps, we find that he comes to grief more frequently when using a radiotelephone than when driving with no such distraction (Brown et al. 1969). Incidentally, the problem is not so much one of inability to drive through a gap of given size, but rather of overestimating one's ability and so being prepared to drive through a space which is too small. Clearly, therefore, one has to be very cautious in applying a secondary task technique, and really it can only be done if the main task is one which can always be performed perfectly, the only problem being the assessment of the amount of spare time the man has available. As long as the limitations of the method are remembered, however, it can be a useful way of locating the times of stress within a task.