The P3b has a positive-going amplitude (usually relative to a reference behind the ear or the average of two such references) that peaks at around 300 ms, and the peak will vary in latency from 250 to 500 ms or more, depending upon the task and the individual subject response. Amplitudes are typically highest on the scalp over parietal brain areas. The P3b has been a prominent tool used to study cognitive processes, especially psychology research on information processing. Generally speaking, improbable events will elicit a P3b, and the less probable the event, the larger the P3b amplitude. This was shown to be true both for the overall probability and for the local probability. However, in order to elicit a P3b, the improbable event must be related to the task at hand in some way (for example, the improbable event could be an infrequent target letter in a stream of letters, to which a subject might respond with a button press). The P3b can also be used to measure how demanding a task is on cognitive workload.