In this report we consider and extend four recent studies of performance data error, report our own study of ASRS-reported incidents, and provide a broad set of countermeasures that can reduce vulnerability to accidents caused by performance data errors. Performance data are generated through a lengthy process involving several employee groups and computer and/or paper-based systems. Although much of the airline industryâs concern has focused on errors that pilots make in entering flight managment system (FMS) data, we determined that errors occur at every stage of the process and that errors by ground personnel are probably at least as frequent and certainly as consequential as errors by pilots. Although relatively few major accidents have yet been caused by performance data errors, our study suggests that more accidents are likely to occur unless existing measures to prevent and catch these errors are improved and new measures developed.