Hidden dependence in human errors

Two methodological refinements to the technique for human error rate prediction (THERP) for adjusting predictions to accommodate unconsidered sources of dependency are presented. (1) Synchrony adjustment estimates dependencies among errors due to cyclical patterns in performing otherwise unrelated activities. Synchrony adjustments would also be appropriate for modeling repairs and component burn-in in systems which are operated in a cyclical fashion. (2) Common rate adjustment adjusts estimates for multiple errors within an activity to make them consistent with the THERP assumptions about the distribution of error rates. Synchrony adjustment can result in substantial increases in the estimate of joint unavailability. While the effects of rate adjustment are less dramatic they suggest that the probability of events involving very many errors is higher than anticipated. Solutions to these two problems are presented along with numerical examples