Effects of varying the threshold of alarm systems and workload on human performance.

The purpose of this research was to investigate the effects of varying the threshold of alarm systems and workload on human response to alarm signals and performance on a complex task. A receiver operating characteristic (ROC) curve was selected to reflect the sensitivity of the alarm system. The threshold of the alarm system was manipulated by changing the value of beta along the ROC curve. A total of 84 students participated in experiment 1 and 48 students participated in experiment 2. Participants performed a compensatory-tracking, a resource management and a monitoring task. As expected, results showed that participants responded significantly faster to true alarm signals when they were using the system with the highest threshold under low-workload conditions. Results also indicated that changing the threshold of the alarm system had a significant effect on overall performance and this effect was greater under high-workload conditions. However, contrary to expectations, the highest level of performance was achieved by setting the threshold at a low level. Results from both experiments revealed that the advantage of faster alarm reaction time as a result of increasing the system's threshold was lost because of its increased probability of missed events.

Behavioural implications of alarm mistrust as a function of task workload.

The research was conducted to investigate the effect of increasing primary task and alarm workload on alarm mistrust as reflected by alarm and primary task performances. A total of 126 undergraduate students performed a complex psychomotor task battery three times, with the number of concurrent tasks increasing each time. During their performance, the students were required to react to an alarm system (including visual and auditory components) of questionable reliability. Depending on the group to which participants were assigned, the alarm presentation rate constituted a low-, medium- or high-workload condition. Alarm response data (times, frequencies, accuracies) and primary task data (tracking error) were analyzed to assess performance differences as a function of primary and secondary task workload levels. Results generally supported the hypotheses: increasing primary task and alarm task workload degraded alarm response performance. Also, response frequencies supported earlier research suggesting that participants 'probability match' their response rates to alarm system reliability. The results are discussed with regard to the cry-wolf effect, attention theory and alarm system design.

Human probability matching behaviour in response to alarms of varying reliability.

The goals of this research were to substantiate the existence of the cry-wolf effect for alarm responses, quantifying its effect on operator performance. A total of 138 undergraduate students performed two blocks of a cognitively demanding psychomotor primary task; at the same time, they were presented with alarms of varying reliabilities (25, 50 and 75% true alarms) and urgencies (green, yellow and red visual alarms presented concurrently with low-, medium- and high-urgency auditory civilian aircraft cockpit alarms). Alarm response frequencies were observed and analysed, and t-tests and repeated-measures MANOVAs were used to assess the effects of increasing alarm reliability on alarm response frequencies, speed and accuracy. The results indicate that most subjects (about 90%) do not respond to all alarms but match their response rates to the expected probability of true alarms (probability matching). About 10% of the subjects responded in the extreme, utilizing an all-or-none strategy. Implications of these results for alarm design instruction and further research are discussed.